It sounds as if you're tired! I wonder why! :^)
You're probably doing better than you think you are. But do yourself a favor, please, and talk with your doctor. You want to make sure there's nothing else getting you down. Being pregnant is fatiguing all by itself. But you know that.
If it helps you, quit reading magazines and books about parenting unless they're recommended by someone you really trust. And don't even come to this site too often. Too much information can keep you up nights.
You don't have to be a perfect mother! You just need to do the best you can. Don't compare yourself to anyone else, but when you meet a real-life mama (or grandma) whom you respect, pick those women's brains.
The best thing I read - and it was years ago - about a large family was a comment of a grown-up child: "There were ten of us kids, but with our parents I never felt like one of ten children. They made us feel as if we were ten 'only children.'"
You probably already know that the simpler your home routines can be, the less stress you'll have. So just do what you can. With four, everything takes more time, but you also get more efficient. Ask your husband what are the most important things to him - because you can't do everything - and concentrate on those. Take advantage of any opportunity for household help you can get - even if you can hire a high school student for an afternoon a week to catch up on what you couldn't get done.
Thank God every day (or twice or three times a day) for each person in your family. Be sure to SEE each child, and your husband, every day - don't just look at them, actually SEE them. Listen to them. Find the good things about each one of them - and about YOU! Change laughter from something you wish you could do - make it a top priority on your to-do list. Look for the funny things, the little things that can be overlooked when there are dishes in the sink. This all may sound silly, but it's a real way of taking care of yourself so you can take care of your family.
My four are grown up, and I regret my fatigue and short temper and wish I had done better. But I did the best I knew how. My children, in turn, are finding out now what it's like raising children!