As far as 4s, I have all of them in the game. I have maxed all of them except Boomer, Kash, Agwe and Gobbler. Those heroes are either redundant to other heroes or I honestly cant see using them in any situation.
I have 2 Jackals and while Purple is ruling wars, I would say its a good idea to have 2-3 of them.
As far as depth, they come in handy. Its nice to have a large toolbox so you can create the best B or C team you can for wars. It also helps a lot on 4* tournaments.
Edit - Forgot to mention costumed TC heroes. They are a huge bench upgrade as many are better than Atlantis or event heroes. Costumed Rigard kick my maxed Khiona off my main purple team and LiXu has been very welcomed in the last 3 wars. I wasn’t going to max Gormek, but I just completed him in case his future costume is unique.
As far as war, they are not needed that much right now. Depending on when season 3 is released and what heroes it brings, I can see possible green tanks after this summer. We have a green HOTM tank coming in March (I think) and then another round of Yunan this summer. If season 3 is release and has another green tank, you can see Green become like purple now thanks to Kunchen and Ursena.
I recommend having a balanced bench to handle changes to the game. I know it’s hard. I can max 2 5* reds right now, but I am holding our for more darts to get another yellow.