Question
Oaks, maples, any such.
Assume 5' tree, lowest branches 2.5' (at trunk),
and that those same branches will remain forever.
Do we expect lowest branches' final height to be
approx 2.5' ?

If significantly higher, then I need to know more.
Objective is lowest branches finally at 4';
and I do not know which branches to leave
on little trees in order to achieve it.

I want to remove unnecessary low branches very early ,
because I have discovered the amazing boost
to young tree growth.
Conversely, I want to leave low branches that
are destined to be at 4'.

Answer
Couple of things and I am following the ANSI A300 Pruning guide here.  Also following Edward F. Gilman's "An Illustrated Guide to Pruning - 3rd Edition".  

A) Branches never rise any higher as the tree grows, where they begin they end.  If a branch starts at 2 feet, it will always be at two feet.

B) Never remove more than 25% of the foliage or leaves in any single pruning year, the tree grew those leaves to provide photosynthesis and essentially its food source.  It knows what it needs and pretty much will grow that much, so do not get overly aggressive as the tree will attempt to compensate quickly in other areas and sometimes in an unnatural growth manner.

C) It is proper pruning practices to leave lower limbs, with the intention of removing later in the trees life, these are known as temporary branches.  Quoting Ed Gilman - pg-139 "Lower branches, called temporary branches should not be removed too early because poor growth and from, a weak root system, or a weak trunk could result.  More root and trunk growth occurs when these are retained, and they can provide protection against mechanical injury and sunscald on the trunk when planted in the landscape."

D) Do not remove a limb in pruning that is larger than 1/3 of the trunk diameter.  (exceptions to this rule occur such as when the limb is broken in a injury, storm, etc.)

One needs to determine the lowest horizontal limb (or scaffold branch) that will be permanent and keep that one in mind.  Any branch below that point will either be removed now or left in place a "temporary branch" and removed a later date"

What is the most important objective is what is known as "formative pruning", working the tree to ensure that when mature the proper form is obtained.  The natural crown, only one (single) dominant stem, etc.  (co-dominant or two dominant stems are common with some species and yours are on the list). ( a stem, leaders or trunk can be used interchangeably.

A co-dominant stem occurs when two or more stems of essentially the same diameter grow from the same location in the tree. Co-dominant stems can be, and often are structurally weak and prone to failure.  If the co-dominant stems are large their failure can cause serious property damage or personal injury.  Removal or repair late in the life of a tree is difficult and costly.

http://www.umass.edu/urbantree/factsheets/35codominantstems_rev1.html  

Suggestions - do not remove too much foliage.  Work towards proper form and structure of the tree.  Remove the very lowest limbs if there is enough left above, but keep in mind if the tree is only five feet tall, there is not much above the two foot mark at this point.  A tree, as in any other life form needs to start somewhere, removal of too much too soon will not cause the tree to grow in a natural manner that you would want and expect.  

If you have a way to send to attach a photo of the tree(s), I would be more than happy to suggest what might be done at this point.  If you wish to keep in touch, I would be more than happy to provide some coaching as the tree grows and matures.

You are more interested in form, structure and final appearance than in speed of growth.  If you just planted the tree, do not expect too much from it.  We always say the first year the tree "sleeps", the second it "creeps" and the third it "leaps".

Young trees can be trained to grow as standards with a 1-2m (3?6絝t) trunk.

Where trees grow with a clear central-leading branch that grows upwards ahead of the other branches, it is important not to cut this central leader, as this could spoil the final shape of the tree.


Some Tips Although Not in Book Is a Rule of Thumb in a Nutshell - but do not over ride any of above information.  The following are only guidelines and may need to be modified for tree species, location, etc.

Never prune a tree the year you planted it other than removal of dead limbs.

One year after planting or second year.
Prune away all side branches from the lower third of the main stem - on a five foot tree that would be approximately anything below 1.5 feet.
On remainder of the tree remove any dead, diseased or damaged growth.

Third Year.
Prune away all side branches from the lower third of the main stem.  This should be somewhat higher than the year before.
Remove any branches on top part of tree which are crossed, misplaced or beginning to become co-dominant.

Fourth Year.
Follow the same steps as for year two.

Fifth Year.
Clear the trunk of side branches to the height desired - this would be your four foot level.
Continue as in step two - with removal of any crossed, misplaced limbs or any beginning to become co-dominant.

Good Luck - and Keep in Touch.