Ahh... now that's another good point right there.
Instead of looking at it against it's competition, try it against it's predecessor and see that stacks up. Then ultimately you can make the comparison of Z4 vs 718 if you end up finding the Z4 better. Then try to justify the price gap
Even if I liked the Z4 slightly better in terms of driving, I'd still go for the Z5 and make the necessary changes to make it suit my preferences. I wouldn't want to buy an older model if a newer model is out lol. That alone justifies my price gap from buying a Z4.