Loss One of my piggies died... how to care for the other one

I would would start looking at shelter for a neutered male, follow the quarantine time and introduce them. At 4 she still has many years left in her. Was Betsie sick? Was it sudden? Watch for signs of Baylie being sick too.

I am sorry you lost Betsie. But I agree with @lunarminx, you should get another piggy. And if Betsie was ill keep an eye out on Baylie or take her to the vet. And it might help to disinfect the whole cage to get Betsies smell off it.

I don't trust the males. The practice of neutering guinea pigs is unusual and not common where I am located. As a result all males I've encountered have been unneutered.

I think pairing an older female with a younger female is best. This has been the best pairing for me in terms of lesser dominance issues and dealing with neutering and spaying costs. I have rabbits which I do neuter and spay. Unnecessary surgery I can avoid is preferable.

I'd get another pig, but it doesn't have to be a neutered male. They're hard to find in most cases. And not everyone is up for living with boar stank.

If you're looking to work yourself out of the guinea pig business, then get another female about the same age as yours. If not, then a younger one (or more) would be good.