considering you said you can record, sending a mic directly to the mic-in of your sound card, but not when you connect your 4-track to the mic-in.. the only thing i can think of to check, is the line-out levels/volumes of your 4-track, and check your cable, that, i'm assuming, translates from a 1/4" out of the 4-track, to the 1/8" mic-in of your sound card..
another thing to note, is that the mic-in's of a cheap sound card, generally out-put in mono (even if it's only receiving a left-channel signal, it will out-put left and right mono; whereas, the line-in will only out-put a left-channel).. just might be something to take in consideration. i have a compaq presario, but i also have a line-in (a cheap sound card also), and i've never tried using a mic-in, as a line-in.. so i know very little of how it works, and what you have to do, to make it work--
gl
(as i reach the end of this post, i glance back to your post, which says "I've checked to make sure the mic volume is up on the computer and it's not muted, but no luck.", and i wonder if you have the volume for recording turned up and _selected_, rather than just the playback volume.. if not, in your Volume Control, go to Options/Properties, 'adjust volume for _recording_', and make sure 'Microphone' is checked at the bottom, and click OK.. check the 'Select' box under the mic controls and adjust the volume.. if that was the case, i'm wondering how you got decent recording with a mic directly connected to the mic-in..
just a thought-- gl)