My MRI showed lumber spinal stenosis. But what is prominence of anterior epidural veins at c3-4 & c4-5 mean? Thank you.

Might be nothing. Prominence of the epidural veins is a non specific finding. Sometimes it is prominent if you have an epidural space infection. Sometimes it can be from venous congestion. A careful review of your images by a neuroradiologist might shine more light into your condition - ask if the radiologist was a board (certificate of added qualification) certified neuroradiologist.
Sometimes. A syndrome called intracranial hypotension can do this. It is caused by a leak of cerebrospinal fluid, usually from a procedure such as a lumbar puncture but sometimes from epidural anesthesia. Vascular malformations (tangles of vessels) can do this. However, it can also be a normal finding. So, your history and physical have to be correlated.