EAX3 showed up with Audigy. EAX4 was an Audigy 2 arrival. The entire line of real Audigy 1-4 cards (non-crap SE/LE/Value editions) runs off of the same driver package from Creative. You should be able to get EAX4 from your Audigy 1. But I've never owned or used an original Audigy so some Googling may be required on that topic.
The biggest differences between the cards are in their circuit board layouts. There are small quality differences between the top A1, A2 and A4 cards. Probably imperceptible. They all use very similar DSPs. There is some difference in how the Audigy 1 and 2/4 do 24-bit output. I believe the A1 can't do 24-bit at all. There was a big stink about that back in the day because Creative mis-advertised. 24-bit source audio is something that is super rare anyway. The value of it is actually questionable in general.