I am a big fan of 4in1 multi planting. Its worked out great in my tiny California yard (zone 9b). I am going to be doing a lot of multi plantings soon… but have 1 concern!
Sometimes 1 out of the 4 may die or may need to be replaced… Do you just dig it out and stick in a replacement next bare root season? The roots may all be intermingled by then, you may damage roots of the other trees in the combination…
Another option I thought about was to use root barriers like in the pic below → the + is the root barrier and the circles are the trees… hope my diagram makes sense… 
I feel this makes it easier to replace any one tree in the combination…
Thoughts? Will this work? I worry the roots may grow lop sided and not anchor well… but on the other side it may encourage deeper roots that is a good thing… Also may be stake the trees? could be an overkill…
Do these root barriers have any chemicals in them, can I use them for organic gardening? Any other similar organic products I could use for this purpose?
Thank you!