Personally, I would be creating additional .pst files and start filing into separate categories. I usually create a new .pst for each major client or project and only have the ones I currently need open in Outlook.
But that size limit was 2 gig, so your 4 gig file that you can't read is not due to this.
Your issue may be that you are using a FAT32 file system (4 gig file size max) or your PST is actually corrupted. The other posts already mensioned some utilities you can use to try to fix your file if it is corrupt.
@ doncarriere: Here you can try Stellar PST File Splitter software which Split Large PST File (of even 4GB) into many small sized PST files on the basis of Date, Time, Email Ids & Folders. The software supports Outlook 2007 & earlier versions.
Download the trial version of this software from the company's website.
I am using Office 2007 and my outlook PST file is just over 4GB and I can't read it anymore.
What can I use to recover the file?
Has Micorosoft fixed this limitation in Office 2010?
Your problem is almost certainly NOT a limitation of Outlook; most of these other posters aren't really offering you a good solution.
I agree with the poster who suggested the most likely problem is that your hard drive is formatted FAT32 - which is limited to 4GB file sizes. To find out go to Windows Explorer, right-click the root of your C: drive (or whatever driove the PST file is on) and choose Properties. The dialog box will tell you if it's FAT32 or NTFS. If it's FAT32 you'll need to convert it to NTFS, which is pretty easy to do.