1. We open a flea market every 4th Saturday.

Question

2. We open a flea market every 4 Saturdays.
[Are both okay? Can we use both sentences?]

Answer 1

#1 is best. We would not say "every four Saturdays". And, remember, numbers under eleven are always spelled out. "4 Saturdays" and "4th Saturday" are incorrect. "every fourth Saturday" is correct.