Cape Town - The Proteas are on the verge of beating Australia in a Test series for the first time ever on South African soil, but they still have one more hurdle to get over.

The fourth Test in Johannesburg starts on Friday, and if South Africa can avoid defeat they will win the series.

With the Australian side in disarray - Steve Smith, David Warner and Cameron Bancroft have been sent home - the advantage surely lies with the hosts, who have begun to find some form with bat and ball.

Dean Elgar, Aiden Markram, AB de Villiers, Hashim Amla and Quinton de Kock have all made valuable runs at different times in the series while, in the bowling department, there has been success too.

Kagiso Rabada remains the most dangerous threat while Morne Morkel looks to be in the form of his life as he approaches retirement. Vernon Philander has been as accurate as ever and Keshav Maharaj chipped in with a breakthrough spell in the third Test at Newlands.

On the surface, this Proteas side looks settled.

But, as is usually the case with a Test at the Wanderers, there may be reason to mix things up.

The wicket in Johannesburg almost always offers the seamers something to work with, and there is often the temptation to go into the match without a specialist spinner.

That was the case earlier this year when South Africa left out Maharaj and India dropped Ravichandran Ashwin.

Should they opt to go that way, South Africa could bring Lungi Ngidi back into the side. But, if they want to bulk up in the batting department, uncapped allrounder Wiaan Mulder becomes an option.

The 20-year-old has been in the squad throughout the series, working closely with Gibson and the management team.

During day four at Newlands, he took to the field during the tea break to bowl some balls on one of the practice strips out in the middle.

Mulder is still incredibly green on the international stage, but coach Ottis Gibson seems to have enough faith in the youngster to back him in a series-deciding match.

"We’ve made a bowling change in every game and in a three-man attack it’s nice to freshen at least one bowler up," Gibson told media on Tuesday.

"We rested Morne in PE and when he came back in Cape Town he was fresh and bowled brilliantly well. When Lungi missed out in the first Test, he was fresh and also bowled well.

"We’ll see when we get to Johannesburg.

"Wiaan Mulder is improving all the time and he will definitely come into the reckoning in Joburg.

"It’s his home ground and we could need another allrounder there and a fourth bowling option.

"There are quite a few changes we could make, but we’ll look at the conditions when we get there."

The Proteas could also opt to leave out a top order batsman for an all-rounder if they wanted to keep Maharaj.

The likeliest to miss out if they went that route would be Temba Bavuma in a move that would see Quinton de Kock moved up to No 6.