Do you see anything wrong when you look at that foot/ toe? If the nails haven't been cut in a while, they will curl under even to the point of digging into the soft pad of the foot. I would think by 4 days it should have improved. Take a good look to be sure there isn't any abscess/ infection/ swelling on the foot. Does it look like it is healing?
Guinea pigs sometimes get an infection called bumblefoot, (ulcerative pododermatitis) usually in a front foot. The picture below is what it can look like. It is pretty hard to get rid of but it can be done. It is usually caused by an injury where the skin is broken and germs get in and make a swelling. If this is the case you can try treating it by soaking the foot in warm (not hot) water/ betadine (you can get it at a pharmacy or evenWalmart) solution, 2-4 times a day. Keeping it clean and dry is very important, so good job there! Some people wrap the foot with vet tape but the pigs don't usually like it and take it off quickly so it seems like a waste to me. It is better to keep the cage very clean while waiting for it to heal. If this is bumblefoot, it will take 2-3 weeks to heal completely. Keep up the soaks until the wound closes and then you can discontinue them. Make sure there is no infection under the scab though, or it will continue to grow and find a new way out. Do keep the cage scrupulously clean until it is better. If you don't see improvement within a week or so, you might have to take him to a vet for antibiotics or other treatment, but I'm with you, it is often traumatic for these little guys to go to the vet. If you can treat him at home I'm sure he would appreciate it.
