[piggiesinpa]

Your pigs are 4 months and 9 weeks? They are young right now so they don't bicker. When they start hitting puberty you will see a change in that. The 2 year old has already went through this phase so you will not have to worry about that, so regardless if you get Toby or not, you are probably going to go through a small period of irritation when your younger pigs hit puberty. It is always a possiblity that you will have to separate, but if you introduce correctly and have PLENTY of space (3x5 is awesome, if you can do more that would be even better) then you are doing all you need to be doing.
The WORST that can happen is that you will have to separate (which will more than likely be the pigs you already have because they have yet to hit puberty).
If you don't want to do it, then don't. The shelter will understand, but explain their situation. You could always ask if you can 'try it out' and if it doesn't work out for you then you can go from there. A local shelter here will let you do a trial basis and/or fostering until you get the 'right' pig. They care about the health of the pigs they are releasing as well as the ones you already have.