Exhaust pipe bender, as used in custom muffler shops. (?) 4" might be a little large, but there must be similar machines out there. I just looked. Used machines start at about $6k-$8,000 US, most are limited to 2 1/2". You will need a pipe bending service for up to 6".

Google it for local providers.

You don't say what kind of pipe? What spec?

Exhaust pipe benders are for thin wall tubing, don't know if it'll work here.

May be impractical, but you could fill with water and freeze it prior to bending.

BTW, this is how they make double wall, bent exhaust tubing for autos.