That means with a 10th level character with a 10th level prestige class is epic level and has to use epic advancement rules?
And then does the epic attack bonus start? "the character’s base attack bonus indicated by his class continues to increase after character level reaches 20th. However, the character does not get mor then 4 attacks for having a high enough base attack bonus. "
When you're at 10th base class level and 10th PrC level, you're only level 20. Just at the edge. The next time you level up, you will be an epic character. You are now eligible for epic feats and PrCs. However, you cannot take epic levels in any class until you have completed all the non-epic levels for that class Ditto for PrCs.
If, for example, you were a fighter13/assassin7, you next level could be fighter or assassin, but you would have to take 7 more levels of fighter or 3 more levels of assassin before you qualify for take epic fighter or assassin levels. During those extra fighter levels, you can take epic feats when you normally qualify for feats, but must take the standard non-epic fighter feats for your bonus feats.