I used to have a z3 and would always get a wave back from a 3, since driving a z4 it's about 50% who wave, the other 50 think your mad! Suppose a z4 is more of a daily drive than a 3 if that makes sense
Back when I had an E30 Alpina, I would always get a wave back from other E30 owners, but with them being s classic, the numbers were low that it was naturally another petrolhead behind the wheel. When I was driving my Subaru Impreza P1, I would always get a "Scooby Wave" , but then only petrolheads are going to drive them.
With the Z4's, I've not had anyone wave back to me and I've come across about 8 or 9 of them in and around Oxford now, which is very odd as generally people don't drive enthusiast cars here, due to the political attitudes a University City breeds. I would have thought E85/86's would now be ending up in the hands of real fans of the car, rather than them being bought because they are cheap and have a BMW badge on them? I see more E46's around these days, as they are genuinely an utter steal, even a 330Ci can be had for under £2000 in really decent condition. Sadly, this also means tons of them have ended up in the hands of some right Chav's , who seem to have ram raided Halfords with the amount of "Modification" that is slapped on to their cars.
I can understand Z3 owners being more friendly, as it's going to attract the same type of people that love the Mk1 MX-5, which ironically is also labelled a hairdressers car by people who know feck all about just how much of a pure drivers car they are. Bar spotting a Z3 on the M40 at the start of the year, I've not actually spotted one since I got my E85 M at the end of November last year. Thinking about it, around 75% of the Z4's I have spotted, have also been genuine M's like mine and they are still selling well above that magical £2500-£3000 price range that put them within the reach of the type of people who like to pretend they are way more wealth than they have, as all they care about is having that BMW badge to impress their neighbours etc.