Dane Swan (thigh), Darren Jolly (knee), Brent Macaffer (foot) and Nathan Brown (knee) travelled to the US on Monday with club sports science director David Buttifant.
Swan has been carrying thigh soreness in recent weeks while Jolly has not played since the Anzac Day clash with Essendon.
Macaffer has been heavily restricted this year owing to foot soreness while Brown had a knee reconstruction in February but holds out hopes of playing again later in the season.
Malthouse said he had been in contact with the travelling party "through the power of the internet" and had been pleasantly surprised with how they had handled the scheduled progression in their training loads.
"They're going very well. Surprisingly for the couple that have been out for so long, their improvement has been very rapid so it's a bonus for us," he said.
"What's surprised me out of it, and David, the training regime for a couple of those players has been moderate here, and perhaps because we've been cautious and not really pushing them through.
"Once the medical staff had agreed they could up their tempo and actually get something from the training, then we took them over and that was in the last week.
"The rapid improvement has come about because we just got the okay for them to step up their training."
The Magpies are fans of altitude training and have held pre-season training camps in Arizona for a number of years.
This is the first time they have taken a small party of players overseas mid-season after initially floating the idea two years ago.
Swan, who was the only one to have played seniors in recent weeks, will miss his first game for the season on Monday when the Pies meet Melbourne at the MCG.
