Answer: Depending upon the rootstock, you can expect your dwarf tree to grow to about 6' tall at maturity. The nice thing about having four different varieties grafted onto one rootstock, is you won't have to worry about pollination from another tree! At this point you shouldn't have to do any pruning.
Fruit trees (including apples) should not be pruned much the first few years to encourage earlier bearing. What you need to do is correct any bad growth habits before they get a chance to become a problem. For instance, you do not want any bad crotch angles--that is, you want the branches coming out from the trunk as close to horizontal as possible, rather than forming a narrow "V" with the trunk.
You want your tree to grow evenly--not leaning or with too many branches to one side. And, although you don't wantyour dwarf tree to get too tall, you also don't want to have branches growing too close to the ground, so remove branches along the lowest 18" - 24" of the trunk. All early pruning should be directed to shaping the tree into the form YOU would like it to be.
The "Modified Leader" method is probably the best route for a home grown apple tree. It starts out with one leader, but as that central trunk becomes stronger it is allowed to form several tops. This helps the tree sustain the weight of the fruit. Eventually, you'll want to shorten the tree a bit, which will make it easier to pick the apples as well as allow sunlight and air to get into the central part of the tree.
As far as when toprune, I would suggest waiting until the very coldest part of the winter is behind you, but the weather is not warm yet. It's a time you're anxious to get into the garden and there are many pleasant days when it's fun to do this. The tree will still be dormant with no leaves, so it's a good time to be able to stand back and make a good judgment about just what needs to go or stay.
When you prune, the first thing you do is remove damaged, broken and diseased branches. Next, remove any branches that rub or touch each other. When that is done, stand back and STUDY from all angles. Then carry out the suggestions above for the 'modified leader' method.
If you do a fair amount of pruning it will produce a big, flush of growth in the spring, therefore be on the lookout for suckers and water sprouts (vigorous vertical shoots) and take them out when they appear. When you cut, cut quite close to the nearest branch or trunk leaving just barely a "neck". Do not leave stumps as they will be a weak point on the tree and invite disease and insects. Remember that pruning off too much at any given time will probably stress your tree. So don't go "overboard".
Here is the web site for The Virtual Orchard "http://orchard.uvm.edu". There might be some additional information there which would be of help.
Q&A Library Searching Tips