I do an interesting comparison of the different kinds of 4d on my web page. Go to
http://www.geocities.com/os2fan2/gloss/index.html and read the several pages i write on the "Hyperspace". One sees there things on time, width etc as the fourth dimension, and how one should read hyperspace.
In practice, what 4D means is the treating of R4 (that is, an ordered set of 4 numbers) as if between any triplet, or any space of triplets, there exists an E3: that is, if R3 -> E3, then R4 -> E4.
This is different to space-time, which i discuss at depth on the Time page, including notes on relativity, etc.
W