Prerequisits for Prestige Classes


Hi all,
I wonder if anyone can help me with the prereqs that a character has to fullfill before switching to a prestige class?

The problem I have is that I'm uncertain if a character can take a PrC and use the increasements from the PrC itself to gain access to it.

I'll try and explain it (I know how I would express myself in Swedish, but I'm having trouble thinking it up in english, so bear with me).

Take the Ka'Toc master PrC in the Narn book p.168.
It requires that a character has Balance 4+, Concentration 4+, Family Ka'Toc, Weapon Focus (Ka'Toc) and BAB 5+.

Can a character pick the PrC when he hits lvl 5? Assuming he has the skills and feats? Since he'll get BAB 5 when he gets the first lvl in the PrC.
Or does he have to wait until after lvl 5 before he can take the PrC, assuming he has a Class that gives him +1 BAB every lvl?

I would really appreciate some feedback on this one since I'm not that used to the d20 system.
I have a couple of players that wants to switch to a PrC really soon, they're lvl 4 now, so it's either one or two lvls left for most of them.
DMG p.176 states that
the first step of advancement is always choosing a class. If a character does not meet the requirements for a prestige class before that first step, that character cannot take the first level of that prestige class