Hi Anne -
We would both like to see the Isle of Capri but I am concerned because
I do suffer from pretty bad motion sickness. Is this something I need
to be concerned about ? I'm just not sure how rough the seas are.
Sea activity can change from day to day so pick one with less rather than more wind. Still, the high-speed ferries only take 20 minutes from Sorrento>Capri so not a lot of time to feel queasy. If really concerned, take an OTC nausea remedy. The BIGGER thing I would be concerned about is motion sickness on the coastal roads. They are very twisty, and I know my own stomach would have been miserable thus didn't use the SITA buses. I was green just in a van from Sorrento to the Naples airport. Should have taken something...
As well, I'd skip the Blue Grotto; the rocking motion of the rowboats can bother weak stomachs plus it's (personally) the one tourist trap on the island. There are better things to do.
Pompei or Herculanium
I'd personally choose Pompeii but you really can't go wrong with either. Pompeii is VAST: much larger than Herculaneum, and it's closer to Sorrento. Then again, with 4 FULL days to work with, you could take one day to do Herculaneum and the archeological museum in Naples, and 1/2 of another day for Pompeii. Spend your other 1/2 of the day doing easy exploration of Sorrento itself. That leaves you a day for Capri and another day to explore a couple of locations around the coast. I'm sure you'll get some suggestions for a few of those from other posters.
We spent 3 days in Sorrento and two on Capri in early October some years ago and everything was open and quite busy. A stretch of really foul weather can temporarily shut down ferry services but that would be the only potential complication I can think of.