Growing From Seed - When To Prune?

I have 5 oak trees (red oak, not totally sure) I started from seed and
they are more than 4 years old. When they were less than 3 months old
I trimmed the tap root and this is all I have done. Presently they
range in height; 8" to 14" inches and with the exception of one, they
all are simply straight trunks and are presently budding (leaves are
probably and inch long) with 3 and 4 leaves at the top of the trunk.
One, however, has two branches, it looks exactly like a Y with budding
leaves on each of the branches of the Y and they are about 1/2" in
size. They are currently (Feb 14th - Maryland) kept indoors, in the
summer I keep them outside. I believe they need to be pruned but I
can't seem to find clear instruction in terms of how and when
exactly. Any suggestions would be appreciated and of course, I assume
responsibility for any outcome per anyone's suggestion(s). The 5 of
them appear very healthy at this time. Help!

When did you bring the trees in? After they went dormant? They should stay outdoors year round. But about your question, pruning slows growth, so you really want to prune when the trees are the size you want them to be. Are you fine with the size of trunk the trees have? If so, you should be fine pruning now. If not, I would plant them out into the ground or a larger pot, then letting them get bigger without pruning.

Hi Allen,
I do wire my oak seedlings in a early stage to get some movement in to the lower trunk.
To get some good taper i would leave the lower branches on the tree and cut each two to tree years the top.
As Ryan wrote, it would be no problem to cut it now
It depends a bit from individual tree to individual tree.
If they are potted, i use pond baskets who are in the ground, doing so i can place them frost free in case i wired them.
Do not forget the root placement, it is as important as the branches.
For some maturity in the tree, you need some more years though.
regards, Sunip

Pictures would help.