I think SA playing 4 pacers says more about what they think about Pakistan's batting against pace rather than what they think the Cape Town pitch will be like.
Pakistan could play a spinner, maybe even 2 as Shadab and Yasir are both quality and I'm very skeptical of the current SA side's ability to play qualiy leg spin BUT.....for them to be effective, the Pakistani batting has to make sure the match actually goes in to day 4 and beyond at least. It's about as hot as it's gonna be in Cape Town during this tour so maybe with the grass drying, the pitch may crumble.
In the first innings even, I can see Shadab being able to control runs on one end to allow pacers to rest but maybe not Yasir.
But even then, if Hasan Ali plays instead and the pitch has some dryness, then I think he's the best exponent of reverse swing in this line-up so even playing 4 seamers won't be such a bad idea but it all hinges on the batting stepping up.
Mickey Arthur should be having a good look at the pitch and thinking hard about the role spin could play.