dear diane,
first of all nice to meet you!
as for your travel, the best to get to essaouira from marrakech is the bus! you have to main options: CTM and SUPRATOURS (both bus stations are close to the railway station in the new town of marrakech), paying 70-80 dirham per person (around 7-8 euro) one way
travelling by car with a driver is much more expensive: calculate 450dh (one way) plus the fuel (around 200dh) and, honestly, it is not so worth while having a driver who can stop when you like all along the way, there is not so much to see ;-)
my concern is only about the 4 days in essaouira: the town, quite small, can be easily visited in an afternoon, and the beach is not so enjoyable if you don't surf / windsurd / kitesurf.. it's very windy, and the water is very very cold! on the contrary, the surroundings are very interesting, and you can easily make one-day excursions leaving from essaouira.. but, for doing that, a car may be a very good idea: without driver, you can find it at 250dh per day (excluding fuel, of course)
in order to have an idea of what you can do in the surrounding, pls have a look at our proposal at the following link -> http://www.yallaz.net/tours.php?dest=Marocco&l=en&topic=intro&scope=excs&id=1
if you are interested in knowing more, pls write to me at monica@djemme.com
hope to be useful for your travel organisation, pls don't hesitate to contact me for any doubt!
enjoy
monica