Your 4 year old son most probably has a throat infection which has resulted in swollen and painful submandibular lymph nodes ("two knots just under his jaw line").
The usual cause of swollen painful submandibular lymph nodes is a throat infection. A viral throat infection will subside on its own in approximately 2 weeks with rest, lots of fluids, and nourishing food. Motrin for children is effective in reducing inflamed lymph nodes and their pain.
A bacterial throat infection should be treated with antibiotics. Look at the back of his throat with a flash light and if it appears red and irritated, then you should take him to be examined by his pediatrician tomorrow who will swab his throat to culture the bacteria so that an appropriate antibiotic can be prescribed.
I wouldn't subject him to an ER environment. It's much better to take him to his usual pediatrician who will be better able to treat him and then follow-up with your son.
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