Bantiekeeper is absolutely right about the problem of introducing just one chicken to a pair. How big is your run? Space is really important in a run designed for small numbers of birds, because there's just less space for a new girl to run away into, even if the run is technically the recommended area for the number of birds in it. To help prevent undue disruption, you will need to offer at least 2 sq. metres of run space per bird, so to introduce one more, you will need at least 6 sq. metres and to introduce another pair to the 2 you have now, you would need at least 8 sq.metres, (more would be better.)
Also, what sort of coop have you got, and how big are the hens you have? Of course sufficient coop space is also important, but it might be that, if you had a big enough run for 4 birds, then small to medium-sized hens might possibly find enough room in the coop, depending on the perch length available. It would be easier to introduce another pair of birds to the two you have now, I've just done it with a couple of new pullets with no trouble at all. But if you feel you really couldn't go up to four now, then you will have to decide whether to try going ahead with a single new girl now, (problematic, but could be done) or wait until one dies and try introducing a pair then (possibly somewhat easier.) You're quite right to think ahead to when you go down to a single bird, for whatever reason, only then it will be with a left-over hen who at least is mature and in charge of the territory. By then, you can perhaps plan a temporary way of keeping the new ones and the old one separate for a week or two, (netting across the run, and a temporary box for roosting in?) to introduce the new girls gradually. At some point, all of us poultry keepers have to make arrangements to separate a bird, maybe because of illness or bullying, certainly when new birds arrive, and it's a good idea to plan for this so you're ready when the need arises. Whatever you decide, I dont see how you could safely manage any introductions until you've thought this through.