The Bills do not have a 4th round pick in next year's draft. It was one of the picks they gave to the Vikings in the Diggs trade.
In 2021 the Bills have 1 3rd, 2 5ths, & 1 6th round picks to work with. As @MAJBobby says they are holding onto their 1st & 2nd round picks, they have a conditional 7th round pick to be determined from Carolina from the Marshall Newhouse trade, so that may not even be an option.