4 days in St Ives area sounds great - depending on your activity levels/interests, St Ives itself (shopping, eating, beaches), a day at Sennen (including walk to Lands End) - possibly on open top bus which runs from St Ives to Penzance and is great. Porthcurno/Logan Rock is lovely (and has Minack Theatre if you like theatre). Lots of amazing (if tiring) walking to be done in Penwith. Lizard peninsula is a bit of a drive from St Ives, but lovely. Gwithian (beach) and Godrevy Lighthouse... Getting to be too much stuff, isn't it!
Haven't decided yet! Was thinking about going and seeing some beautiful places. I didn't thnk wold do all of it in 3-4 days. Just two or three spots. Just as an example went to Wales last year and saw portmerion, snowdon, and Beaumaris. Was thinking of doing same.