There are a ton of things that you can do to incorporate learning into your everyday
Babies around that age usually love the outdoors and can learn so much from their surroundings. When my daughter was little like that (and, of course, still now at 20 months), she loved being outside. My husband and/or I would carry or wear her and point everything out to her. Clouds, trees, grass, flowers, etc., and then we would take her up to the flowers so she could smell them. She loved this, so we did it almost daily. We would say the name of the flower and say smell the flower. We would point out the birds and say listen to the birds chirping, we would show her squirrels etc.
As she got older, we began signing each of the things we pointed out to her which she promptly learned. Your son is only 4 months old, so it will likely be several months before he starts signing back. Sometimes they start signing back at 8-9 months, and some babies start signing back after 12 months. Even though they don't always sign back when they're younger, just know that they are taking all of it in and learning. My daughter had a signing explosion after she turned 12 months. All of a sudden she would sign things I had taught her previously that I didn't realize she knew. I started with only 5 very useful signs when she was very young (milk, mommy, daddy, potty, water) and incorporated more over time.
In addition, you can do lots of tummy time and playing on the floor. This helps really strengthen the muscles she needs for great head and body control. It strengthens the muscles she needs to roll over, crawl, etc. Also, when we did tummy time, we did a lot of playing. I used to hold a toy within her reach to encourage her to grab it, and then I would hold it ever so slightly out of her reach to encourage her to reach for it. Baby safe mirrors are great to start using at this age too.
I wore my daughter in a wrap, sling, or carrier a lot! This was a great way to involve her in what I was doing, whether it was cooking, folding laundry, or anything that I was working on. I always explained what I was doing, and I talked to her about our day. This is a great way for babies to bond with you and learn about practical life, and by doing this they get to hear so many words by being talked to all day.
You can also gather different types of fabric (ie. silk, burlap, cotton, corduroy, crochet, etc.) and make a little book that he can touch and feel the different textures for sensory play.
Rattles and toys like maracas that he can shake to make noise are great too. He can learn cause and effect this way as well…"when I move the toy, it makes a noise"
During bath time, I would always tell her what body part I was washing. I would say or sing each body part. She learned her body parts very quickly by doing this with her at every bath time.
Aside from these things, there are things you can do to help him learn to read and develop his vocabulary. Many people start showing Your Baby Can Read DVDs at around this age and work through the program. Also, of course, there is Little Reader which is a more comprehensive software program that is very highly recommended. I love how it not only teaches sight words, it incorporates phonics and helps expand their vocabularies exponentially. Little Math and Little Musician are two other programs that are highly recommended. Little Math helps with subtilizing quantities to 100 and then even teaches addition, subtraction, etc. Little Musician is wonderful as well. All of the BrillKids products have fairly short lessons so you don't have to worry too much about screen time. In a couple of months (or maybe even now) you can start showing some of the Preschool Prep DVDs like Meet the Colors, Meet the Shapes, Meet the Letters, and Meet the Numbers). PP also has Meet the Sight Words that you can show as well..
I almost forgot to mention a few of my favorites!
1) Sparkabilities…it's fantastic. You can get the app and buy the lessons for only $20 (if I remember correctly) or you can purchase the DVDs for a bit more. They have different DVDs for infants and through the toddler stage. It's a right brain program that I found to be invaluable. My daughter absolutely adores it. I highly recommend it!
2) Tweedlewink…this is pricey, but I love, love, love this program. it's also a right brain program that is essentially flashcards on DVD. It covers everything from math dots to science to art to speed reading and more. Also, there is photo eye play in the DVDs. Each lesson is only 8 minutes long, which is really great if you're trying to keep screen time to a minimum. You can start the lessons over at several different stages in the child's development, and they take something new from it.
3) Signing Time and Baby Signing Time…Love both of these series!!! I like Baby Signing Time to start out with and then working toward Signing Time later on (after baby starts signing back more or after 12 months). I credit teaching my daughter sign language with her ability to communicate her needs/wants/feelings at such an early age even before she could talk. At first, I was teaching her signs myself only, then we started Baby Signing Time which she loved. This is most definitely one of her favorites.
There are so many wonderful programs out there. I would recommend just picking a couple of things for now and you can incorporate more later on.
I think this has become a really long post! I hope it helps though
Good luck on you and your sons's early learning journey!