say on the fourth day, you do nothing. can you hold the 4:1 overnight again even though your SMA is at $25K? what if on the fourth day, you sold $50K of stocks and replaced it with a different $50K of stocks. you are still at 4:1. can you hold this overnight?
SMA is not reduced until you put on new positions or withdraw cash. If you closed 50k in the first stock you would receive a 25k credit to SMA. The new purchase of 50k in stock would reduce SMA by 25k. No net change.