We spent 2 nights at Grootbos in early part of October. We were in the Forest Lodge which had just re-opened after being burnt down in the February fire that swept through the area. It is a spectacular place both for location and the accommodation - we had a lodge which can sleep 4 easily. Well furnished & equipped e.g wood-burner for cold nights, private deck & o/side shower.
As to the time of your visit you should be ok for whales (see Dyer Island Cruise boats web site) Grootbos reserve seats for you & minibus you to the dock. We had an amazing trip early in the morning seeing whales all around us, seals, penguins & a 10/12ft Great White close enough to touch! It was a beautiful flat calm sunny day. Next morning's trip was cancelled because of the poor weather so the guests hoping for that trip were out of luck. The weather can be unpredictable as your other correspondent has said & as August is still end of winter/start of spring it may be even more unsettled.
The guides/staff are keen,knowledgeable and friendly and as far a s we were concerned they could not do enough for us. The food was great.
I am sure you have looked at their website & do e-mail them for info on trips etc available at that time of year - they had a very busy programme available when we were there & it is good you are there 4 days as 2 days is a bit hectic. They were happy to answer my queries about the effect of the fire etc.
I agree with the other correspondent about Cape Town - it is too far for a day trip in my view, and I am a keen driver and made use of the empty roads to go quite long distances at times. Car hire is really essential to get about due to lack of public transport and flexibility & personal security.
We had a great 16/17 days in SA and everywhere we went we had good experiences but Grootbos was one of the highlights (especially with the whales to watch) - however, if the weather is poor it may not be so good and 4 days might seem a long time!