Well what do you want to do.......?
With only 4 nights it kinda excludes to much travelling around, but if you could fit it in hire a car and head down through the High Atlas to Ait Benhaddou just south of them for a quick taste and to realise you need to go in the Toyota
In Marrakech well theres loads to do, head to Djemaa El Fna the main square and watch the entertainment, it's out of this world at night, loads of eating places although the ones with a view over the square charge a premium. Chez Chergrouni at the left on the way into the souk is good, cheap and quick. The Marrakechi a few doors along is 5 Star or eat at ones of the stalls in the square.
Spend some time wondering around the souks enjoy the shops are prepare to get lost at least once
Other recommended sites
Majorelle Gardens - renovated by Yves Saint Laurent are great even for a non gardner.
Saadian Tombs
Marrakech Museum
Palais Bahia and el-Badi
Take a Caleche Ride (Horse & Cart Taxi)
In fact pretty much the main sights any guide book references, you can really do most of the main sights in a couple of days in comfort
Something I've not visited but will next time I'm in Morocco are these
http://www.atlasobscura.com/places/dead-trees-marrakech
Any questions ask away I've been travelling Morocco for 15 years now