E.B.
I agree with the Mall walking and the water outside suggestions. I would stress routine. Not sure how old your child is now but routine saved me when they were little. A sample of our typical day was after breakfast we left to do SOMETHING.. errands, groceries, library. Back home for nap and lunch, then SOMETHING, drawing, craft, sidewalk chalk, hide and seek, SOMETHING..... Then bath, dinner and reading before bed. It helps you both knowing what to expect. Good luck. The reason things are getting to you is because it's new and there is no structure. Make structure. Good luck and please don't worry. Your time with him is precious. Cherish it!
Okay, just now read he's 4 months old and your situation....in that case my SOMETHINGS would be, rolling a ball to him when he's on his tummy, giving him plastic spoons and cups to reach for and pick up. 1 inch of water in a outdoor plastic pool so he can splash and have some fun ( with you right there of course) , carry him around with a kangaroo pouch so you can do household stuff. I did this and we talked, I let him feel things talking about textures, colors etc. Naps will be a big part of his day still. Just involve him and GET OUTSIDE whether early or late in the day. Fresh air is important to you both! God bless...