I understand things get hard & its very unfortunate for you to be in this position i wish you the best with your marriage & ur family. I do not think you should abort based on your relationship, yes your are a mother of 3 already & yes it will be hard at times but when your husband is gone & all fails your kids will always be there for you & im sure your child would grow to love you & your 3 kids will grow with age & be able to assist you in raising this little one. I am 19 years old & a mother of 1 expecting my #2... I am struggling in many ways as well but i think abortion isnt the way. maybe adoption? or just being a single mom & working it out..
good luck & best wishes with your decision.