What you describe is highly unusual. Your employer should have included the $4750 in box 1 of your W-2 and withheld FICA and income tax.
You said "I guess $5250 of that tuition assistance is tax-free." That may not be true. The fact that your employer is doing it wrong indicates that he may not have set up a qualified educational assistance plan. In which case, the $5250 is not eligible for exclusion The entire $10K may be taxable to you. Try to get clarification from your employer. Reference: https://www.law.cornell.edu/uscode/text/26/127
Either way, enter as described by @SabrinaD2 .
