whaas, do you plan to overwinter the container in the garage or something like that?

I wonder how the tree form paniculata would do in a container over time?

Any plant can be grown in a container provided the proper conditions are met and the correct longterm care is provided. A tree form hydrangea can get pretty top-heavy when in bloom, so plan accordingly for wind. As 25G container is very good sized -- I'm not sure I' be overly concerned about winter and potential damage on such a hardy plant. And it is possible to winter protect a container in situ.

Given your choices, I'd go for it!

I planned to keep it outside all the time, so I was looking for something that was zone 4 hardy.

I'll probably give the paniculata a shot...yes first container plant for me, lol.

I can't seem to find anything else that may bloom in part shade and get at least 5' tall.

This is probably morphing to a container garden post but this one should be quick.

I drilled the holes (its a textured dark bronze plastic pot) and added 2" of 1" stone. Is that sufficent or do I need small "pebble" like stone?

Also I can just use a typical potting soil mix for the media right? My experience has been paniculatas aren't very picky.

No stones! Or gravel or pot shards or anything else at the bottom of the pot. Contrary to common belief, adding stones or gravel at the bottom of a container does not improve drainage - it actually slows it (complicated reasons involving a perched water table). Just fill the entire container with a good quality, highly textured potting soil. Most commercial bagged potting soils are too finely textured for longterm container culture - they don't hold up very long and 'collapse', reducing aeration and hindering good drainage - but you can improve on them by adding bark fines and pumice or granite grit. Or visit the CG forum and look at the recipes for the "gritty mix", which is intended for woodies in containers.

I'd also recommend adding a slow release fert into the soil before planting as well.

In about 3 years you wil need to root prune and repot your hydrangea with fresh soil.

Hmm...didn't know that about the stones.

I just looked at the tag. It said to drill several holes on the side, 1" from the bottom and fill just over that point with pebble stone.

I'll poke around in the CG forum and check on some recipes. I did just buy this product from Shultz (Tree and Shrub soil) it seemed to be light and fluffy...and what looked like bark fines. I wonder if I could use that?

Looks like I have more research to do...the root pruning and replacing soil is all new to me.