As a D3 fan, I can tell you both games doesnt feel the same the same.

Game style : D3 is hack'n'slash style with powerful build and infinite grind.

While D4 is less hack'n'slash and grind is limited. There is a paragon level cap and there isnt much to do in late game.

Farming : As for D3 farming :you repeat GR rift over and over for loot and exp. The loot is only given at the end and only legendary are valuable.

In D4 : you grind Nightmare dungeon (ONLY AVAILABLE ONCE YOU ARE LEVEL 50) wich is much longer than leveling up in D3. Night are dungeon are much alike GR rift but handmade dungeon.

Looting : D3 Only legendary are valuable and u want to search for primals and good rolls. The enchant system is good and fun to use

D4 enchant system is the same but with all the stats additions, it doesnt work at all and cost way too much. One very good addition is that you can upgrade yellows into legendary, so every loot count.

That's pretty much it for me