Hi Piet,
NPL (WCM) licences require a Class 4 medical....a PPL will require a Class 2. (jy gaan daai viets bietjie langer moet trap!

)
The good news is though, that you are of course allowed to fly your trike on the Class 2...you do not need to do a Class 4 as well.
When I did my Medical, the quack gave me a class 2 (I was also contemplating doing my ppl at the time.) Although the Class 2 has now expired, it is still valid as a Class 4...and is valid for the same period.
From CAR'S:
(b) Class 2 –
(i) private pilot: aeroplane and helicopter;
(ii) student pilot;
(iii) cabin crew member; and
(iv) free balloon pilot for commercial purposes.
(d) Class 4 –
(i) microlight aeroplane pilot;
(ii) glider pilot;
(iii) gyroplane pilot for non-commercial purposes;
(iv) airship pilot for non-commercial purposes;
(v) free balloon pilot for non-commercial purposes;
(vi) hang-glider pilot;
(viii) paraglider pilot;
(viii) powered paraglider pilot for non-commercial purposes; and
(ix) air traffic service assistant.
(2) A flight crew member who holds a valid Class 1 medical certificate referred to in subregulation
(1)(a), shall be deemed to hold a valid Class 2 medical certificate referred to in subregulation
(1)(b) and a valid Class 4 medical certificate referred to in sub-regulation (1)(d).
(4) Upon expiry of a Class 1 medical certificate referred to in sub-regulation (1)(a), such
medical certificate shall be deemed to be valid for the remainder of the period for which it would
have been valid as a Class 2 medical certificate referred to in sub-regulation (1)(b) and a Class
4 medical certificate referred to in sub-regulation (1)(d), as specified in regulation 67.00.6.