I heard its sposed to be easier than introducing two males of the same age.
3 to 4 months seems a little old, so I'm not sure, if the current two piggies are bonded. I would leave them that way, you don't want to risk breaking their bond and having 3 single boars, you could try placing their cage next to the older boars so that he doesn't get lonely. I'm picking up a 6 week old boar tomorrow to introduce to my 3 year old older male, as an older boar is more likely to accept a younger boar.
So you could try, but it will take a lot of patience. Seeing as there are two of them it might be a bit harder. Try searching the forum to see if you can find any tips on introductions.