Yes, iPhone 8 is slight heavier than iPhone 7. Apple iPhone 7 is 138 Grams while iPhone 8 is 148 Grams.
Though, Apple has not mentioned on their website about the same, but during our extensive review of the device, we have found that iPhone 8 64GB & 256GB variants does have 2GB of LPDDR4 type RAM.
Yes, you can record 4K videos with iPhone 8.
Yes, iPhone 8 does support fast charging.
No longer the gold standard, the iPhone 8 is still all the phone you need.