Speaking to New Zealand media on Tuesday after the long journey from Johannesburg, Rennie confirmed changes were always part of the plan for the series closer, with the need for fresh energy compounded by the absence of three key players.
"We sort of always thought we would make changes in the fourth Test regardless of whether we won that third one or not," he said.
"Make sure we are bringing in guys with plenty of energy and share the load. Obviously with three key players missing, it alters that thinking as well. But what we know is we will have a highly motivated 23 guys stripping at the weekend.
"We have definitely got one more in us."
The All Blacks and Springboks travelled together on the same charter flight from Johannesburg to Washington, DC and Rennie said the shared journey reflected the goodwill between the nations, even if the atmosphere was more practical than playful.
"There was a bit of work being done, there was a lot of sleeping going on, and yeah, it was fine. I'm not sure how much bantering or that sort of thing went on, but there's a lot of goodwill between the two sides, so travelling together was good."
Looking ahead to the contest, Rennie zeroed in on the Springboks' set-piece strength and the need for greater accuracy and continuity from his own side.
"Yes, I mean, we could talk about scrums, but we'll have to be better," he said.
"They've got a great scrum, they've got a great maul, so our ability to stay in front of that maul is important. And when we did that, we limited their ability to hurt us, and likewise at scrum time.
"The easiest way is to limit those occurrences. So, if we're more accurate, there are fewer scrums and greater continuity for us, and we don't kick the ball out.
"There's less line outs for them, and discipline comes into it. You can't just make their strengths disappear. But our job is to limit the amount of opportunities they get.
"I think on our ball, the scrum was reasonably solid. We got it badly wrong on one, then got punished with one of their feeds, and so look, it's a weapon for them, and that's the challenge.
"We want high continuity. If you make an error, they get to go to a set piece. They get a penalty off that. They march up the field, and so yeah it's still an area we're going to have to be better at.
"South Africa are very good at creating opportunities and taking their chances. So we know if we can do a couple of things a bit better, limit their opportunities, and be a little bit more clinical, the result could go another way.
"But they're a very good side, and they scramble really well, and yeah I think the better side probably won in the end."