A major web development problem used to be that it was not possible to run two or more versions on the same computer.
A solution has been found however, by web developer Joe Maddalone who deserves eternal fame. Read his page for the technical details.
Web developer Ryan Parman made the customized browsers files available on his site. I copied them to my server to make sure this valuable resource doesn't disappear. Subsequently I found similarly doctored Explorer 3 and 4 versions on nachtarchiv.de.
Here's how it works:
IEXPLORE.EXEfiles in each directory. This works. Nonetheless, keeping the versions apart is not simple.
@importstatements that a single-installed Explorer 4 doesn't allow.
All these browsers work perfectly, with the exception of Explorer 4. Explorer 4 refuses to accept any URL you type into the location bar. Instead you should open a file by Ctrl+O and entering the URL in this box. Clicking on a link works fine.
Another problem you must know about: when you use conditional comments the comments will be interpreted the same by all browsers. Generally this means that if you have Explorer 6.0 installed, conditional comments will pretend all Explorers on your computer are Explorer 6.0. This means that it's impossible to test conditional comments for Explorer versions below 6.0
My old method, maintained mainly for the record.
When you start with a clean Win98 First Edition installation with Explorer 4, you can install this full Explorer 5.01 next to it. It's a bitch to download (66 Meg), but I can guarantee that it works because I've done it.
Be sure to choose Custom Install. One one of the installation screens you see an 'Advanced' button. Click on it, and you see a screen with a checkbox 'Keep Explorer 4 in compatibility mode' or some such text. Be sure to check this checkbox, then continue the installation. It works.
Years ago I used the same trick on WinNT, and there it worked fine, too. I have heard, but not personally tested, that you can install Explorer 5.5 in a similar way. Note that you must choose between 5.0 and 5.5, though, installing both next to 4 is impossible.