PS2 version????

FINALLY got PS2, I need to know what version I have have and what to buy. I bought the GT3 bundle and the serial # is u6115891. From what I've been reading the "u6" would mean version 6, but I haven't even heard about about a v5. Is my system a v4???? and can I use neokey and GSv1.6 on it to play backup?????