I had sex and came on my period the next day, everything seems normal and I'm on my 4th day of period, but could I be pregnant

Did you use a condom... if not, then why not?

In answer to your question... yes it is ALWAYS possible to get pregnant, the question is how likely, which can honestly range from very likely to very unlikely. Without knowing your exact ovulation cycle... (not the same as your Menstruation cycle) there is no possible way of giving you what your true odds are. NOTHING is 100% certain other than abstenance. Not the Pill, Not Condoms, not a combination, and certainly not the "Get on your knees and pray you aren't" method.

i had sex and came on my period the next day, everything seems normal and im on my 4th day of period, but could i be pregnant

Possible, but not likely. As smoothy said, there is no 100% safe time, but sex right before a period is due is one of the least likely times to become pregnant.

Most women ovulate towards the middle of their cycle, about 12-16 days before the next period, but it can happen where some naturally ovulate sooner and some later. Not common, but it can also happen where a woman may have an off cycle where ovulation occurs outside the norm for her.

Always use protection correctly every time you have sex to decrease the odds even more.