Personally, I'd spend the four days on Skye. But I think that a lot is going to depend on mum. If you take the tour out of Oban, you're going to be on the boat for about 10 hours. I don't know if the boats provide landing on Staffa in March. Perhaps another reader can provide that information. It's quite possible that the boat trips to Staffa don't start until April. Best to check the websites for the sailing season.
I'm assuming that you'll have a car. If so, you could take the CalMac ferry to Mull, drive out to Fionnphort, and take the three hour Staffa tour from there. You'd also be able to visit Iona, if you so decide. If the tour boats don't sail in late March, you could always just visit Iona. It's not as spectacular as Staffa, but it's definitely more spiritual!
A lot is going to depend on weather. It can be pretty wet and wild in late March, with even a bit of snow thrown into the mix. On the other hand, you could have sunny weather, with highs in the upper 50s (Fahrenheit). Again, check out the tour boat schedules. CalMac will be running the ferries, albeit on a more limited schedule through March 26th., so you should have no trouble getting to and from Skye and Mull.
Best wishes for your holiday. Be sure to bring some layered clothing.