Hi there! I think your Morkie is adorable!! Now that she's 4 and you don't plan to breed her, I would suggest it is best to spay her.
The reason is (even though Rufus is fixed) - she could accidentally get loose and be mounted by another male (and a male that is too big could really hurt her). Also, there is research showing that spaying helps prevent certain types of cancer and
totally prevents uterine cancer (due to the nature of what spaying is).
I believe it is best for her health so I would say to spay her
