It look like the stood the drywall up so you have a vertical seam every 4 ft. The take a little more effort to hide than the horizontal seams.

You could likely make it better with mud but i will leave any instruction for the people that do it more than me.

 

Start by holding an 8' straight 2x4 across the area, be sure it is very straight and maybe longer. Then you will see the high and low spots to help judge how much correction is needed.

Personally, if it is just drywall (not plaster) I would remove a large area, repair the studs, then install new drywall. Except for the extra tape and mud time that is a one hour job.

But use the straight piece of wood first to see if added mud is possible. And as Neal said, the pros will be along.

Bud

 

Like Bud said!!

 

1930 home my guess is it was Plaster that someone removed & replaced with drywall. There are many ways to make the wall true.
The straight edge is the quickest.
A string line pulled from corner to corner in many locations.
The dot & screed method which can be used vertical & horizonal @ the same time.