Yes, you can play 4K videos on iPhone 8.
Yes, iPhone 8 is slight heavier than iPhone 7. Apple iPhone 7 is 138 Grams while iPhone 8 is 148 Grams.
No. iPhone 8 does not have the FM radio.
Yes, iPhone 8 does have the OTG support.
No longer the gold standard, the iPhone 8 is still all the phone you need.