Hi
First of all I don't know you, but with your age and experience with raising one younger child, you are really putting a lot on yourself. The 8 yr. old should be in school during the day the 5 yr. old might be in pre-K & the 3 yr. old. The 5 month old. Wow you are really expecting a lot of yourself. Second, PLEASE don't put the impossible expectation of doing this perfectly or never losing control. You are stepping in with 4 kids who have a different set of disciplines than maybe you do.
Third, If you are watching them in your home, it's a strange place to them, even if they have been in your home many times. They are feeling as confused as you are right now.
Have you tried taking care of them in their home and not yours. They will have their toys, movies, etc. And the comfort of being home. Also saves you from the transporting items back and forth. Which add a great deal of fatigue.
Fourth, the 8 & 5 yr. old can be encouraged to help during a time the family needs to pull together. Even the 3 yr. old can do a few things with you.
The most important thing is your 7 yr. old didn't sign up for any of this. Keep an eye on him and make sure he is not overwhelmed by it all also. He can see what his mom is feeling and can become very concerned for you. He might even act out. Yes the situation can't be helped. But if your boyfriends kids & grandkids have friends ask them to help, if the mother has family call on them. Try not to put this on yourself to the point where it causes problems. This is also a time of the year that I'm sure in the last 3 yrs. your son and you had some very special talks and time for shopping. It's difficult for him also.
I know as a grandma of 3 under the age of 3 that came to us one at a time, how much strength it takes. Thinking about 4 of them being placed on me with a 7 yr. old of my own, is stressful in itself. So PLEASE be kind to yourself and do what you can without making the next few weeks miserable for you and your son, which in turn will make things miserable for these 4 little ones who miss mom already.