I've got experience with this very thing. I put 25 chickasaw plums in the ground back in March 2020. They were bareroot seedlings from our DNR. I put them in 5 foot tubes knowing they likely wouldn't like the tubes. Those trees have done tremendous over the last 5 years. I took the tubes off last year. They are all out of the browse range of deer and have been that way since about year 3. About a third of them have been producing plums for 3 seasons now. Some of them are just loaded this year. They are very fast growing in tubes and have really started to sucker this season. But, the 5' tubes did deform their natural growth. It pushes limbs and sprouts that want to grow out up instead. In some cases, the limbs and sprouts could be gently pulled back down to a more natural position and in some cases they were just too mature to do so. I cut the top 2 or 3 feet of the tubes down the middle at year 3 to try and give the limbs some room to expand out - it helped a little. In 2021, I added a bunch more plums and put them in cages with short pieces of tube around the base. That way I could still spray up against the trees to control weeds while not having the tube constrict the outward growth. That worked just fine but didn't result in as rapid growth as the 5' tubes. I wouldn't hesitate to try the 4' tubes in your case but I'd also get those expanding limbs room to grow outward within year 2 or 3 if you can somehow. Again, I was successful with some of them in getting the tree above browse height and pulling limbs back down some but there is certainly a time when growth and maturity is too far along to do this. In my case, even though some of them are somewhat deformed as a result of the tube, they are all suckering real nice. I wanted a border screen/thicket like you referenced here and the limbs that are deformed from the tubes haven't really negatively impacted my overall goal at all. I've had the same experience with American plum too. I love plums for habitat. And the bucks seem to love them for rubbing as soon as I remove the freaking tubes!!! I've put some tubes together to make a larger diameter tube too. It's an improvement on my original effort but I still think the best if you have the time and resources is to put a shorter tube on them inside a cage if your goal is a nice looking tree. If you just want a fast thicket started, tube them and don't worry too much how they look later. Good luck with the project.
