Premiere Pro very likely will not be able to read your old PrElements PROJECT file, but you should be able to open your existing video files... BUT, that may depend on the codec of your files
If you don't want to spend the "about" $800 for Premiere Pro CS6 (more for a suite) you could always buy Premiere Elements 11 for "about" $100
Of course, PrEl-11 is also not likely to read your old PROJECT files... but should work with your existing video files
You also need to be aware that PPro CS6 REQUIRES a 64bit computer and operating system http://www.adobe.com/products/premiere/tech-specs.html
If you do buy PPro CS6, you will need to ask questions in that forum http://forums.adobe.com/community/premiere
As John T. points out, the older PrE 4.0 PREL (Project) files will likely not open in PrPro CS 6, though it will be worth a try. PrPro, can Open PREL files, from versions that are somewhat coincidental, i.e. PrE 4.0 Projects WILL Open in PrPro 2.0. They were released at about the same time. Later versions of PrPro have trouble with older PrE Projects, but part of that can be that PrE has some different features, than does PrPro. For instance, there are no Themes/Instant Movies in PrPro, so if one has any of those, all bets are off. Same for some 3rd party Transitions, that are in PrE, but not in PrPro. Also, many of the Effects, even with very similar names, are very different in PrE, than they are in PrPro. If the Project is very simple, then it well might work.
If you want to create Menus, you will want Photoshop also. The Production Premium Suite for CS 6, has all of those programs, and it's really like buying two, but getting another 3 programs, for free.
If you do go for the CS 6 suite, for PrPro, go to the PrPro FAQ sub-forum, and read the threads there, plus be sure to see this entry: http://forums.adobe.com/thread/878529?tstart=0
After Effects, Photoshop and Encore also have their own forums, and they are very good, and very active.
Ok, but my project consists of making a bilingual DVD with a menu. I know Encore can do so, but can Premiere Elements or PrPro CS6 can do so?
Adobe Encore, only available with PrPro, can do that. You can do multiple Audio Tracks (up to six, IIRC), selectable via Menu, or the DVD/BD player's Audio button. You can also do true Sub-Titles in Encore, though you will want a Sub-Title writing program, to get that part of the Project started. The rest can be done in Encore.
And, of course, if you're interested in saving about $1000, Sony's DVD Architect Studio only costs $39 and makes a great companion for Premiere Elements. It creates very complicated, multi-level DVD and BluRay menus.
But I guess it depends on what exactly you want to do and how much it's worth to you.
Premiere Elements is all in one software, for video editing AND disc authoring
Premiere Pro is video editing only, it does not create a disc... which is why Encore is bundled with PPro