Hi I have a four year old son he breathes with his mouth open at night during sleep and snores a lot some nights?

Time for an eval. The snoring is likely to get worse with time as the tonsils tend to grow faster than the kid at that age. If he hasn't already he can develop a sleep disturbance, sleep apnea, with prolonged spacing between breaths (>20sec) that are a problem. If you can record his snoring & take it to your pcp they will likely send you to an ENT or for a sleep study. A t&a is often curative.
4yr old son. Probably he has enlarged adenoids and tonsils. Please take him to see an ENT specialist. Chronic oxygen deprivation at this age is not good.