West Ham joint-chairman David Sullivan has told supporters a fifth signing can be expected in the next 72 hours.
A number of Championship clubs are keeping tabs on Leyton Orient defender Mathieu Baudry, Sky Sports understands.
Clubs outside of the Premier League have until Thursday to tie up any short-term deals.
McCarthy would like to add before having the option taken away, but admits finding recruits of the desired quality will be difficult.
He will not be dipping into the market for the sake of it, despite being short on Football League experience, as he has full faith in those already at his disposal.
McCarthy said in the Green'Un: "I'd like to bring in some loanees just to beef the squad up, but there's a difficulty in doing that.
"If you bring players in from the Premier League, the first thing those clubs will say is, 'We want them to play' because that's why they send them out.
"If you look to bring players in that are disenchanted elsewhere, well they will obviously want to play.
"They'd have to be really good players to get in the team, so it's not easy just getting loan players just to beef the squad up."
While McCarthy is expecting little movement when it comes to incomings, he admits Anthony Wordsworth - who has found first-team football hard to come by this season - could be on his way out.
He added: "We had an offer from Preston and I thought he was going to go last week. There was a number of reasons why he didn't.
"Preston wanted him until January without a break clause, but that would have left me high and dry after the 14th of December, so I wasn't prepared to do it.
"Is he still available? If it works and it suits, there is a possibility for it."