Here's how to change the number of items shown in the Recent Items list in the Start Menu (not in the folder you're talking about - we'll get to that in a moment): Change Number Of Recent Items On Vista’s Start Menu
. After you do this (try something no higher than 50 or you may be pushing it), you will need to right click on Start and click properties and change the number of items to show (which should now allow you to increase it to 50 if everything went right). It won't happen immediately, it will need to build up the list from when it was changed. First, I'm not sure this will work if there's a problem with your Recent file in your user profile since it essentially gets the information from there (and/or the registry) and if that is limited, this change may not make any difference as the source information may not be available - but it's worth a try (and it will improve how much you see from the Start Menu (if it works correctly)). But that's not what you asked about, so let's talk about your real question.
As far as I know, the Recent file in your user profile has no date limit though it may have a size limit. I could find no information about this file or how to modify it if that's even possible. On my system (32-bit Vista Business SP2), I seem to be limited to 150 items (which seems like plenty) but they go back months (even years) - there appears to be no date limit (I don't open many different files very often). I could find no settings to adjust this file size or set a date limit (and though I could see where all the entries are stored in the registry I could find no registry key to set the storage limits). When you say you can only go back 4 days, how many files are in the Recent history file (is it 150 like mine or some smaller number)? If you add a new recent item, does the number change or is the oldest item deleted to make room for the new entry? In other words, is the limitation based on the date as you suggested or is it based on the maximum number of files as I suspect?
If you have another user on the computer, log into that account and see if it has the same problem - open and modify some files if necessary to test it out if it doesn't have much of a history. If not - if it shows more files and goes back further (you may need a few days to test the date issue if it doesn't already have older items listed), then your user profile may be corrupt. To fix this use http://windows.microsoft.com/en-AU/windows-vista/Fix-a-corrupted-user-profile.
Other that that (and some generic troubleshooting and/or some rather radical options), I'm not quite sure what else to suggest at this point. Perhaps feedback from what you find by doing the checks I recommended and by answering my questions will provide a lead to some easier alternative than a full blown troublshooting process. Perhaps it will turn out to simply be a corrupt user profile and the above procedure will resolve the issue (if there is an issue - I don't know your computer usage habits and you may be maxing out even at 150 in 4 days for all I know).
I hope this helps and either resolves the problem or identifies the problem or determines there is no problem and you are simply maxing out the system (for which we can't do anything as far as I can ascertain). But try the above and see if anything helps and then reply back with as much detail as you can. The more we know, the better the chances we can help.