I've seen many posts that say "make sure to take "x" levels of "y" class pre-epic in order to get "z".
(i.e. take 4 levels of BG pre-epic to get 4 attacks/round).
What is the restriction post-epic with this? Why can't you get the same benefits over level 21?
Basicly the relationship between base attack progression and number of attacks per round ends at level 20. You need a base attack bonus of 16 or more at or before level 20 to get 4 attacks/round. Unless you are a monk, then you use the monk attack progression, but only for unarmed fighting or with kamas.
Hope this explains it.
You get a similar effect with the saving throw bonuses.
Pre-epic the "high saves" get a bonus increase that ends at +12, while the "low saves" end at +6. In epic levels all saves get the same bonus increase: +1 every 2 lvls.
What this means is that your saving throw bonus is determined by what classes and # of lvls you take in them pre epic, while what classes you take in epic lvls has no influence.