While I don’t have any of the expansion packs, I used to read the sims forums at gamefaqs.com quite reguarly. I am pretty sure that they are seperate, which sucks because you have to pay $120 to use them all.
Of course not. They want you to buy all four, so no there are no objects and skins (besides career skins) common to the expansions. On the other hand, the patches are included with future expansions, so that one fixes bugs they forgot to fix before releasing the last expansion, but of course if you don’t have that expansion it doesn’t do anything to your game.
It does strike me very odd, though, that with RTT you do get stuff from the 1st expansion by buying the second: how uncorporate minded of them.
Also keep in mind if you do buy the expansion packs that there is a “recommended” (read required) order in which they must be installed (Sims -> House Party -> Livin’ Large -> Hot Date). If you skip one (say, you have LL and HD but not HP), everything will work fine, but if you then get HP you must uninstall LL and HD and then re-install everything in the correct order. Kind of a pain.
Actually, it’s worse than that. You can’t uninstall just the expansions, you have to uninstall the entire game before you can install them in the “right” order. Most people install them like this LL->HP->HD->vacation. Techically you can install HP before LL, but since they came out in that order, most people who have buying them as they come out have LL installed first. Also, you need the HP disk (if you have HP installed, that is) for part of the HD installation, so you better know where the disk is before trying to install HD- I had to hunt for two hours before I could finish the installation…but like Sublight says, they are worth it.