I'm not aware of any issues with app development on a Hackintosh. I'm not saying there are any, but we use a Hack for all our main work, specifically due to the memory requirements for Genymotion.
We have Xcode 9.3, all the simulators, we use all the certificates from Apple, we compile and upload from Xcode to the app store, all the command line tools. We tend to develop using the command line, emacs and write our own tools, but thats because of our development style. I know that many people use Xcode (pile of utter junk IMHO) rather than an other IDE. Not had a single issue in three years.
The only two applications that I am aware of for us (YMMV) are iMessage, which we know we can fix but can't be bothered and Photos. There is an issue with the background daemon process in the face recognition element of Photos which will not work on a Gigabyte Z97X motherboard. No idea why, but it has to be disabled. Other people have reported the sane issue. We use Lightroom instead.
4GB of memory on an Air is perfectly adequate for software development. We have a Macbook Air as well and have taken it away and done development with Xcode on it. I wouldn't run massive databases on it, or do significant video editing, but a 4GB Macbook Air is fine for a lot of uses. Happy to take it off your hands if you don't want it.
