Definitely a 4 is more desirable. The early 3's had the wood rail angle that that if the rubber goes bad, a modern superspeed replacement does not fit properly. The later 3's use modern rubber. All the 4's and later use modern rubber.
The 4's have better pockets as mentioned above. The 4's got rid of the score counters and the "brunswick" on the end rails, like the 1, 2, 3 have, so less metal to scratch your cues. The 4's look better.
Edit to add, this makes the 4 a better table, but as far as value, I don't thing 1-4 are priced much differently. I think only the 5 is more as its much newer.