Well a few things you could do if you're on Rein is to either swap to a dive comp and either counter dive their dive or dive the enemy backline when they dive yours.

So in this situation you've be diving a Winston and Genji with 3-4 maybe all 6 of your team and more than likely get 1-2 kills and if you do then great but you might lose one of your backline depending on how well you do. Regardless you'd either be even or have a 1 hero advantage to quickly engage on the rest of the enemy team.

The other option is to dive the enemy backline which in the example you gave I'd be a bit 50/50 on doing this because you'd be diving as 4 against 4 with no heals and the enemy would have heals. You might get 1 pick or 2 if lucky but I'd say this option is better if say your backline got dove by 3-4 enemy heroes to give you a better chance at taking their supports out while they're on your supports. The winning side in this situation would be down to who can get the kills fastest, which backline can get out and ult advantage after diving backlines.

In a situation where you aren't in a dive comp I'd say it's really up to your off tank to peal for the backline but that depends on your off tank. If it's a Zarya and you're on Rein you both could rush back to help if you're close but if you're not then I'd say rush the enemy backline mainly if the tank left behind is a D.va while you got a Zarya cause free ult charge (even Hog but more dangerous damage). Your approach to this could be to just focus down the tank, the D.va would either fly away or lose mech and you're up against 4 squishy targets which makes it easier for your team mainly as Mr Rectangle Man with his giant hammer damage.