I'm pretty sure the Z4 ecu can't be mapped, you can fit a piggy back chip, this is what I started with some 9 yrs ago now .
Your best bet is to look for a 2nd hand Power Fc ecu which can be mapped.

The Z4 ecu I belive was a 260bhp unit from the factory .

Silversurfer and jolly green monster are the guys that can give you all the answers.

I would take it easy until you get some answers from these guys, the way I see it, is you've a standard ecu which is not now complimenting your decat .

To give you an idea , I achieved 292bhp with a piggy back chip in a Z4 ECU with full decat and a crude boost control Dawes device .
Then fitted a power fc and 550 injectors and had it mapped by JGM and achieved a safe 336bhp , this was a 96WRX

Power fc will cost approx £400 and injectors approx £100

Hope this helps a little , and good luck with the modding