You say that your tree is already at around 5m after just 4 years. That means that it is on a pretty strong root stock.
Yes, you can keep the tree down to around four metres but it will need a tight control kept on it.
Either in the early spring or in the late summer you will need to prune out any of the branches that have grown very strongly up through the tree, It is no good just cutting branches back as they will just grow all the stronger in the upright position.
Branches that are supple enough to bend will benefit a lot by tying down as far as you can. This slows the sap run and encourages fruit bud formation.
As regards the roots. From the strength your tree is showing already you will have a problem controlling their growth by top pruning.
However most fruit trees have relatively shallow root systems so every few years just dig a small trench in front of the area you are worried about and check for root runs.
A rough guide to root distance on fruit trees is that the roots normally extend to about the same distance from the trunk as the canopy above covers.
An apple a day.....keeps me in work.