The R's have already brought in six new faces to Loftus Road this summer as they look to gain promotion back to the Premier League at the first attempt.
"I know what we need and I still feel the squad is a few players short," Redknapp told BBC Sport. "If you are going to win this league or get promoted you have to be that much better than the other teams. We do need to improve and we all know that.
"We've maybe brought five or six in, but I still feel we are three or four short still of real quality if we are going to make the difference."
QPR were knocked out in the second round of the Capital One Cup on Tuesday as they lost 2-0 at home to Swindon Town.