If I was the buyer I wouldn’t be happy with a delayed completion date of 4/5 months. To my mind you’re either selling something or you’re not, if someone wants a rapid completion I would do everything reasonable to meet that. Perhaps the house has been marketed too early?
I’m in the process of buying a house before mine’s sold, ideally I’d like an overlap if 3-4 months to make changes to the new one, but if someone’s prepared/ able to complete faster I’ll go with it. The shorter the time between exchanging contracts and completing the deal the better IMO.