This is getting repetitive now, but I did as you said.
And a basic search for iOS reviews for Tapatalk 4 shows no major bugs, minor bugs, or really anything negative. It's a free upgrade to the latest version that works just as good as the previous version did.
The Android one is a different matter - but I did specifically say that I've seen no problems using the iOS one. They are completely different apps, made using different languages on different platforms and so one version being crap doesn't make the other one crap too.
To basically sum the topic up:
If you are on iOS the app works fine. Upgrade if you want.
If you are on Android the app is buggy and not worth the upgrade price, so don't bother.