If you look at the whole range of Primula you'll find plants that will cope with your zone, and in a range of heights, colours, and flowering times.

If you don't mind planting out in the spring you might want to consider using polyanthus which come in a wide range of colours. Some flower in a clump on the top of a stalk and others come on separate stems from the plant's rosette.

You could also look at cowslips and oxslips which die away to a crown for winter and then return for spring.

Some primroses will be happy in one position for quite a while; others need to be lifted and moved to a new site - well-fed with compost and ancient manure - or topdressed so the new stems can put down roots and continue with restrained spreading. (They don't tend to go far.)

Provided that you don't get a great blast of heat in spring the basic 'English' primrose will flower for more than a month, and the polyanthus will continue flowering for several months, which may or may not be wanted.

They'll put up with cloudy weather and mild temperatures but start to look tatty in hot weather.

If you were to get some of the 'Barnhaven' doubles you may find that they die away to bare stems for the summer and start to recover in autumn. That, however, is in a maritime environment which generally doesn't have extreme temperatures to either end of the mercury.

You might also want to look at auriculas - but they need annual care to keep them in good order and they don't like an excess of damp over winter.

I don't know anything much about any of the primroses that Vetivert8 talks about, but the ones I grow are pretty much a one season type of plant, and a fairly short season at that. I am not sure if that is true about the polyanthus etc., but it is something that you might want to consider.

I grow the kinds of primula that like moist soil, such as P. bulleyana, beesiana, x bulleesiana, pubescens, etc. since the lower part of my yard is quite wet. Most of these (exception is pubescens) grow somewhat taller than the under 18 inches you are looking for and look best in masses. They also have a fairly early summer bloom time and are done in a couple of weeks. If the soil is wet enough they do keep decent looking leaves for most of the rest of the season, but tend to look dreadful in droughts. They are very pretty for a short time in early summer along the wet pathway, but need to be interplanted with other later blooming plants.

Have you considered any of the short perennial geraniums, such as Biokovo or Carmina (x cantabrigiense), or the very short Grayleaf (cinereum), 6-10 inches or any of the sanguineums, mostly 12-15 inches tall? They make great underplantings, very nice foliage, and like the light shade of taller perennials. In zone 4 you can use many part shade plants this way. Other possibilities would be heucheras, tiarellas, heucherellas, small hostas, lamiums, many sedges (carex evergold is a beauty) & Ladies Mantle (there's even a dwarf LM ( A. erythropoda), if A. mollis is too big).

You also mentioned Sweet Williams. These are not true perennials, but sometimes act perennial in my yard. I sometimes use them as edgers, but they have a tendancy for only some of them to return each year, which can be frustrating. I have had much better luck with dianthus gratianopolitanus, which forms bluish evergreen mats of foliage. Firewitch (2006 perennial plant of the year) is one that has proved very hardy for me, even in my wet clay. It has hot pink flowers, and will do some rebloom even in a short season.

Another possibility for full sun is the coreopsis family. Many of them are quite short, some fairly short lived perennials, but heavy bloomers while you have them, mostly yellows, but some pinks. They make great filler plants and the common yellow ones can perk up a lot of otherwise drab one-color beds with a nice spot of yellow. I also like to use annual coreopsis seed in pots (yellow with red markings, and especially the short mahogany colored ones) - they are quick to sprout and quick to bloom and very cheap as well.

If you have dry spots, you might also try some of the short Iris. Dwarf bearded iris have pretty, fairly large flowers, and quite short foliage, with lots of color choices. I use them mostly for foliage contrast, but the flowers are also lovely. I also use Iris pallida (mid-size) in semi-shady areas for the creamy variegated foliage, although I get some bloom as well.

There are also lots of short sedums which like it a bit dry and work well around lilies and other bulbs, or other plants that prefer to be dry. These are mostly spreaders, can be easily expanded by cuttings, come in a variety of foliage colors & even have some nice seasonal bloom as a bonus. Lamb's Ear is another short underplanting possibility. BTW, it looks great with Firewitch and any purple, red, or black foliaged plants.

thanks for the info. I remember when I had primrose long long ago they faded in the summer. Now I know why. I'll get some in the spring. Are primrose and COWSLIPS related? My mom grew up a farm girl and used to cook cowslips like spinach but you had to get the leaves before they followered. Coreopsis I have and they come back for years but only come in yellow. Sedums I just don't like the look of and Lambs ears I have but they are too invasive. The rest I will look on the web about. I used to have Sweet William that came back for 3 years then died out. Why? Dianthus is a great possibility.

Your Sweet Williams might survive longer if you cut them back after they bloom. At least that's what mine do. I have footings which just went through their fourth year after learning this 'trick' from another gardener. Previously, they didn't last longer than 2 years.

You might also consider Lynchnis Viscaria Slendens Fever. Commonly named German Catchfly. (I find pics on goolgle don't do it justice in case you decide to go there.)
Nice blooms in June, and when you cut back the spent flowers, you are left with a nice tuff of foliage resembling grass - but nicer! Early hummingbirds appreciate them as do butterflies. I've used them for edging one of my gardens and like them.

Here's a pic of a Hummingbird Moth on mine at the end of the flowering peak. I was actually deadheading that day but stopped when I saw this little fellow. You can see the foliage. As the summer progresses the foliage gets really compact and forms a nice mound.

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One really nice plant that you may not know is sedum angelina which looks like no sedum you have seen before. It is a spreader and a wonderful edging plant in bright chatreuse green. Makes a great contrast with heucheras like Obsidian.
Here's a shot of them when they were first planted back in '04. They have both increased in size of course since that time:
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I too grow primulas in damp areas. I tend to prefer the tall candelabra types, but also have cowslips and a variety called Lady Greer which is pale yellow and gorgeous. It doesn't last terribly long, but it is very early and that is a treat.

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I find my primroses spread a great deal into my 'lawns'.

I found this site not to long ago and it is really an amazing primrose site. This woman has a beautiful primrose garden and this is a virtual tour of it. If you are interested in primroses this is really a great site to check out.
http://www.vtprimroses.com

Here is a link that might be useful: Arlene's Primroses

You might also consider finding some nice Dormant mini daylilies. I love the look of them planted with lilies. I know that there are some that would reliably come back in your zone. There is a website with links to sellers of northern hardy daylilies, called Daylilies on Ice. you may want to check them out. They come in a variety of colors, and come back bigger and better every year! You could also check out the Lily auction, and just look for dormant varieties. Not sure if mini roses are hardy in your zone, but I love them too, and they don't get big, and they bloom all summer long. Have fun choosing! -Jessica