Spellweaving is a stand alone skill in that magery and eval has no bearing on damage done. You will do the same amount of damage on WOD at 100 as you will at 120. The only way to do more damage is to get a higher level focus and wear more items with SDI. There are two spellweaving spells that you can cast that will boost WOD damage and that is to be in Reaper Form and cast Arcane Empowerment.
As far as the 4/6 casting, there once was a list that had all the caps and at what level you needed to be to cast WOD at 100%, but that was many months ago and on the old forums. You may find the info on an old Five on Friday.