S3xp and S4xp I don't get it...

[Ultrax75]

I bought a S3XP not long ago joust to find out that Sony has released an even better model (darn!!!) for more or less the same price.

Nevertheless, I have searched the web for a simple answear but I joust can't find it. Why did Sony release the S4xp so closely to the S3xp. Is there anything they had to correct? Or did they joust do it cause they wanted something even better? I there anyone who knows ALL the differences?
So far I know this:
S3XP: 80 GB HD, single layer DVD, 1 GB maximum RAM
S4XP: 100 GB HD, doubble layer DVD, 2 GB maximum RAM.

O and one more question. If I plugg the AC into my portable and the battery is fully loaded will the battery be damaged or worn out if it runs with the AC in for a long time or does the computer completely switch from the battery to AC power?

Thanks for any reply

[torson]

Additionally s4 has 533FSB and uses pc 3200 memory - Whereas the s3 has slower speeds.