Of course you can add more. There is also that same tubing with 6" spacing. http://www.mrdrip.com/ML106-Black-Microline-100-6-oc-_p_901.html

How long are you running them? Also, do you know the flow rate of your system? The tables on this site might be helpful in determining how long to run the system: https://extension.psu.edu/determining-how-long-to-run-drip-irrigation-systems-for-vegetables

The most common problem with drip systems, as Jamie mentioned, is failure to run them for a sufficient amount of time. 1/4" supply lines would require several hours of run time. That is why 1/2" or even 3/4" line is far more commonly used in garden beds such as yours. 1/4" can work well for small containers but not large areas so you will need to do both - add more lines and run them for a much longer time frame.

Dave

Thank you all. They are emitting at 1/2 gallon / hr and I run them for 1 - 2 hours.

I wonder if I should run 4 more vertical lines... or run in a grid pattern.

Don't you think running the 1/4" for 3-4 hours is too much? With sandy soil, you aren't going to get the spread, like you would a harder soil and placement on the ground. Once the water hits the bottom of the bed, it's not going to spread out anymore than it already has. If you run 2 more lines, I would make it so the emitters are in between the current ones. It looks like you are getting pretty good coverage currently, from emitter to emitter. What are you growing?

If I ran 2 more vertical lines, it would be a total of 45 emitters at .5 gallons per hour. 22.5 gallons total per hour. Would 2 hours, every other day be enough for a 10x4 bed? I guess depends on a few factors... I am growing vegetables-tomatos, kale, basil, spinach, radishes, turnips.

I think that would be more of a nursery question. Some things need more water than others. For your tomato plants, you could put a ring around the plant with your micro tubing. It's what I do. Putting a ring with 4-5 emitters. That way I get water all the way around the plant. Smaller plants, I think, can do with just watering on one side, near the plant. 2 hours every other day seems like a lot, unless you are somewhere very hot and dry!

Regardless of soil type the ideal approach is DEEP watering done LESS frequently. Experienced gardeners will shoot for 1x a week - assuming no rain - unless they are growing in the desert or unless well established plants show signs of wilting in high heat.

Frequent watering results in shallow-rooted, water dependent plants that are easily stressed. What shows on the surface is not a valid indicator of soil moisture levels. When beds are properly mulched, and with sandy soil mulching is vital, you can't even see the soil surface. So what counts for plant health is the deep soil moisture levels as that encourages deep rooted more drought tolerant, less stressed plants. You can find many discussions about this here.

The only way to know how deep your chosen time frame of water goes has little to do with # of gallons since that 22.5 gallons is dispersed over approx. 40 cubic feet (depending on bed depth). The only way to really know is to stick your hand deep into the soil, even into the soil under the bed itself. That moisture will wick back up to the plants as needed.

And with mixed crops in the same bed and with no way to zone the watering you will have to water the lowest water-tolerant vegetable - the turnips and radishes - and then spot water some of the others like the leafy greens that need far more water.

Dave