Get away from the kids, or encourage him to take all four of them somewhere for the whole day. Get a break entirely from the 24/7 responsibility. Your love for him is probably still there, just somewhat being suppressed by the constant responsibility for the kids.
My guess would be your intimacy has become a routine, a chore, and a pattern that never varies making it a boring activity in your mind. Because of that possibility, your fantasy with another mate fills the excitement you've lost with your husband. 4 kids to take care of can also make intimacy a chore. You need to communicate more on ways to spice up your time with your husband.
spend time with your husband this about his good thing which make you happy and excited this the good stuff about him