Both are electro optical compressors and the classic applications for them are vocals and bass but can of course be used with success on other sources aswell. That's the similarity of them, the difference is in controls, design and sound. The LA-3 has two modes, compress or limit (the switch is on the backside... why did UA not put it on the frontpanel on the re-issue...????

) and on the front you have two knobs, input and peakreduction. The LA-4 on the other hand has, if I remember this correctly without one in front of me, threshold and ratio controls. more "modern" controls if you like. I'm not that much of a tech but I know the LA-4 uses op-amps and the LA-3 does not. Unfortunately these op-amps (and there are 4 of them I think) are not that brilliant and you can clearly notice a loss of high frequencies when it compresses a lot. I've heard it's a common LA-4 mod to replace the op-amps. Most people choose LA-3 over LA-4.