This is part of the Pattern Day Trader rule. However, you are limited to 3 trades (options and equities, not futures) in a rolling 5 day span not 4 trades. If you make 4 trades then you are labeled a pattern Day Trader and must maintain $25k of equity in your account at all times.
3 trades per week would be 3 trades this week and 3 trades next week, etc.
A rolling 5 day span is 5 consecutive trading days. For example, M-T-W-T-F or T-W-T-F-M, etc. As each day drops off of the current 5 day period, so too do those trades made on that departed day, as applied to the total number of trades made.