not got it on the z4.. don't think I'd put it in the z4, or any other car I'd drive on the road.
When I had the 350z, I fitted Wizard of Nos' SB150i6 kit, configured to give 60hp (but could be turned up to 150).
TBH, I wish I didn't bother with it.
It cost £700 for the parts and would've cost an extra £200 for fitting, but luckily it was quite straightforward on the 350.
Refilling wasn't too bad - the 11lb bottle cost £34 from a local(ish) refilling station.
Downside was the insurance went absolutely sky high and no dealer wanted to service it with the kit on - meaning I had to remove everything and return it to stock before they'd inspect and stamp my book.
It also runs the engine very hot if you use it a lot. If I went for a burn round some backroads with it primed and used it quite a bit, I'd often get home and the engine heat guage would be 3/4 of the way round..
I also had to have the O rings changed after 25k due to the problems caused by the N2O.
Basically, good for a dragger / sprint car. Bad for everything else
(IMO of course!)