To clarify - are you meaning you wish to add more to live with your current two, or two more to live separately from them?
What sex are you guinea pigs?
You can only add more piggies to the two you already have if your piggies are female. If your two current piggies are Male, the you cannot put any more piggies in with them. Any more pigs you get will have to be kept in a separate cage. Boars need to be kept in pairs only. Any more will cause fights. You also cannot keep two boars with any number of females, again it will cause fights.
Also, if your piggies are Male, then you should not risk keeping a separate pair of females in the same room, as the smell of females can cause boars to fight.
If your piggies are female, then you can add another two females to them. Or one more female and a neutered Male.
Your cage converts to 120cm x 60cm. This is the absolute minimum for housing two piggies.
To house four piggies together (assuming your current piggies are female), then you will require a cage measuring 210cm x 60cm - of a cage with the equivalent amount of floor space. Most commercial cages are not big enough for four piggies. The best and most affordable way to house multiple piggies is by building a c&c cage. They can be configured as you wish ( L shaped for example) and are pretty much one of the best ways of getting a cage large enough for multiple piggies. Lids can be added to c&c cages.
Cage Size Guide
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?