It is hard to say without knowing what type of tree/shrub you are talking about. Assuming they have been in a pot now for 4 years, they really do need repotting. Aside from Madrona, which I would never repot (they hate it), I repot conifers every two years if under 15 years of age and I wait 3 to 5 years for older trees. Deciduous trees get repotted every year, unless they are very old (25 years plus)

The repotting season is slipping past quickly so if you are going to do it, do it soon. If they are in forest soil, try to chopstick out 75% or so of that nasty soil and repot into a mix of 1/3 akadama, 1/3 pumice, and 1/3 lava. Sift these to get rid of all the dust and to get like sized particles. Repot and water daily (you cannot overwater this mix). Since this has no organic you will need to use some sort of organic fertiler -- I like cakes best. However wait 1 month after repotting to fertilize.

Good luck.