Be careful with the nipple shields and over using. It makes getting the milk out much harder and can affect your milk supply. I would definitely check for a tongue tie and also, pump for a few mins to enlarge nipples. Also, you might be latching incorrectly. Make sure you are getting enough areola in to push your nipple far back in baby's mouth. Use the "hamburger method" of latching. Squeeze nipple and areola together starting with bottom of areola insert and then nipple and top areola. Also, try to relax, if you are stressed baby will pick up on it and be stressed too. It's not easy but I also would recommend regular supplementation in these early days. It can be a. Recipe for disaster and mess with your milk supply and also your baby might reject the boob because bottles are so much easier to drink from than the breast.
My first did that, she had a tummy ache from being gassy. All your baby knows is their belly doesn't feel good, and eating makes it feel better. Not the case with gas. Gas drops for infants or gripe water can be helpful. At 4 days I would try to stay away from formula and bottles. It can cause nipple confusion and make continuing to breastfeed very difficult. Best bet is talk with your pediatrician and a lactation consultant.
My sisters little one was tongue tied & they didn't pick it up at the hospital. It's the little part of skin under the Tongue & sometimes it doesn't break so Tongue is stuck to bottom of month. It's fairly common & really easy to fix. Worth getting someone to check.
Try getting a nipple shield. (fake nipple like mentioned above.) You can get them at any drug store. It is just a clear piece of plastic that you sit on your nipple. (it almost looks similar to a bottle nipple.) But it worked for me with both my daughters. The baby could also be gassy (depending on what you have eat.), tongue tied... It could be several things. Call your peds dr. if things don't improve. Good luck!
What is a tongue/lip tie?
Maybe he has a tongue or lip tie?
With my daughter I had to use fake nipples, mine were too small for her to latch onto. Sometimes I would pump just to enlarge them to make it easy for her.
try expressing sone milk first than have him latch on he might not be patient enough or is just too hungry to wait for the milk to come out.