Yes, Apple iPhone 8 does have the OIS. It helps greatly with blur in the event of shaking hands or dim lighting and the larger sensor consistently picked up more light for better low-light photos.
iPhone 8 does support fast charging. Apple iPhone fast charges up to 50 percent in 30 minutes when you use an Apple USB-C to Lightning cable and one of these adapters: Apple 29W, 61W, or 87W USB-C Power Adapter.
You will get about 51-52GB as free memory in case of iPhone 8 64GB. However, this is very much dependent on the software version.
Yes, you can play 4K videos on iPhone 8.
Though Apple never mentions the battery capacity of their phone on their website this is what we have found during our battery tests of iPhone 7 Vs iPhone 8.
iPhone 7 has 1960 mAh Li-Ion battery while iPhone 8 has 1821 mAh Li-Ion battery. But thanks to the latest iOS 11 and the new Bionic chip support, iPhone 8 battery lasts longer.
Apart from the battery capacity difference, iPhone 8 also has wireless charging functionality and quick charging which are missing in iPhone 7.
No longer the gold standard, the iPhone 8 is still all the phone you need.