Would you buy a PSP4000 if it has no UMD slot (and so going full DL) and a much longer battery life (like + 40-50%)?

only if they allow me to download the UMD games that i already own for free then sure.

There is only one thing that would make me buy a PSP as things stand (4000 or not), and that is the news of the PSP2 not being backwards compatible with the PSP.
If it isn't, I will have to buy a PSP to get my hands on the games I want to play on that one.

Only if it's possible to install CS. I like my Sega CD games to-go.

If i was to buy a PSP and there was one with no UMD slot but with more battery life or whatever i would buy it. Of course if i owned one i don't think i would buy games for it anyways, just torrent them, or use it for music or playing old games.

only if it has another analog stick and a motion sensor !

No UMD slot = no sale. I don't want to be forced into buying a larger memory stick just to be able to continue playing games.

No, I like UMDs and actually owning the games. If I could buy retail packages with DL codes, and get my old games for free, I would be happy to do so