I ended mensus on 20th of aug, is it safe to have sex on 24th of aug and not pregenant?

With a condom: It is never really “safe” to have sex without condom or birth control. If you are trying to use cycle timing the date your menstrual cycle ends is not a useful marker. Cycle timing has about a 20% failure rate so trying to guess around ovulation does not work well. For those who use it you need to know your ovulation days. There are apps that can help with this. Just use a condom