would anyone know who makes the largest reloading press?I need one for 4-bore rifle cartridges,i know RCBS makes one is there any more?

Redding offers their Ultra mag. I don`t know if it is large enough for your purpose though.

RCBS makes the AmmoMaster. It was designed for the 50BMG, I have one & it makes loading the longer express type cartridges very comfortable.
http://www.rcbs.com/default.asp?menu=1&s1=4&s2=2&s3=10

IIRC there is one made in Australia, and also IIRC it was imported by the Old Western Scrounger. Who is bringing it in now I don't know. A 50BMG on this thing looks like a 22Hornet sitting in a Rockchucker, as it was desinged and built to handle the smaller cannon rounds cannon rounds (37mm and down)

The 4-inch case length is not going to be that great a problem. However, the greater than 1-inch case diameter will be.

Let us know what it is like to put 275 grains of powder in that case, and then top it off with a 1800 grain bullet...

As for the press, there are several turret presses out there that can be modified to work, but if you want something that will do the job without doing the modifications, look for an old RCBS Big Max press.

Dave Cumberland still offers his Rock Crusher press in Dayton, Nevada. It has a 7½" stroke and a 2¾" die station, with 3" available as a special order. The ram diameter is 40mm.
It will serve for 23x115 Soviet.
It weighs 67 pounds.
Cheers from Darkest California,
Ross

Here is a pic of my aussie made "zero"press which would do what you want.

I really miss Dave now that he moved to the sun belt. He occasionally brought his cannon up here to the Simpson range when the club sponsored the bi-annual cannon shoot. It is coming up soon and I'd really like to see his Howitzer in action again.
Frank