You might understand the plot of the fourth game, but you would miss all the jokes that refer back to the older games. There were things like that in the third game, like the clown's red nose.
However, I'm sure you could enjoy the fourth without playing the others first. When the third game came out, they said they made it so that even new people would like it.
Also, I got into Gabriel Knight games through the third and had no trouble enjoying it. Still, I recommend getting the others first if you just have the chance.