We would highly and strongly advise that you do not attempt to add another piggy.
It’s good youve asked first rather than coming here to try to solve the usually disastrous fall out of an attempt.
You are very lucky in having a functioning boar trio. As they are adults (and assuming they’ve been together their whole lives) and have made it through the risky period together, then don’t do anything to put that in danger.
Most boar trios fail and never even make it to adulthood together. They find it hard to form a functioning hierarchy. Not least because the space needed is much larger than normal cage sizing - a trio needing a minimum 300x100cm cage.
If you add a fourth piggy (of any age) you are highly likely to rock the boat substantially.
Boar quartets are even more unstable than trios. You run the risk of fights and breaking bonds.
Even if a baby was initially accepted the chances are that when that baby hit his teens the disaster would strike.
If you choose to attempt it anyway, make sure the cage size is at least 400x100cm (even then it probably wouldn’t be enough to stop fights), and that you have plan b and c living arrangements prepared.
Adding the fourth piggy in may mean your current trio never goes back together as it is. You could end up with two pairs or even worse, it could break all bonds and then you’d have all four piggies all refusing to live together and consequently you’d need four separate cages.
If you want more piggies then the safest way is to have a completely separate pair of boars in a completely separate cage, never having interaction with your current trio.
Also if you do choose to get more piggies, don’t be tempted to have any sows. It’s not recommended to add sows into the same room where boars live. Even if the boars can never get to the sows, simply smelling sows can ruin a boar relationship.
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
A Comprehensive Guide to Guinea Pig Boars
It’s normal for piggies to not like to be touched. Most don’t like it.