Is it safe for a guy to ejaculate inside a woman on day 4 of her cycle given that she has a 25-day cycle?

Usually...: Its never completely safe if you don't want her to get pregnant; however day 4 is quite low-risk. Make sure you are counting correctly. Day 1 is the first day of the period, so most women are still menstruating by day 4. Ovulation usually happens 14 days prior to menses. In a 25 day cycle that would be around day 11. However, sperm can hang around for days. Better to use protection.