In a 4 DPS comp you want to have 1 tank and 1 healer. Those heroes will change depending on the DPS you're running though. Let's say that you're running this:

Hanzo

McCree

Tracer

Soldier

With a comp like this, you're going to want to run a Rein and Mercy because it will allow your Hanzo, McCree, and Soldier to be more effective due to not getting shot at anymore. The Mercy is honestly there for the Rein. Due to running 4 squishies, rezzing them on time won't be to hard either.

If your comp looks something like this:

Genji

Pharah

Tracer

Widow

If your team is running this comp then you want a Winston/Lucio/Zenyatta. The Winston will help to take down squishies which in turn will allow Genji to clean up and the Zen would allow the Pharah/Tracer/Genji to stay alive much easier and kill targets much quicker. If you're running a Lucio it will give your team more sustain because you're able to heal more targets and not get hit as much.

TL;DR: Is your comp more divey? Run Winston and Zen/Lucio. Is your comp more stationary? Run Rein/Mercy.