Doing a smpte or midi time code sync'ed transfer with a 4 track cassette machine would be WAY too complicated IMO, and to do so would require that you give up a track on the 4 track machine for the time code to be on, as well as more than likely also needing a more capable software program to do it right, in the end, I think it would be a lot of needless effort in order to do a 3 track transfer, besides, if you already have 4 tracks recorded on the cassette machine you won't have a track for the timecode.
Yes, if you did a stereo mixdown into the computer you would lose the ability to do any mixing inside the computer.
After doing a little bit of looking around, this is the card I would choose for the task, the price has come down quite a bit, I seem to remember this setup selling for over 200 dollars not too long ago, this is a really good deal IMO.
M-Audio Delta 44 $149.00
http://www.americanmusical.com/item--i-MII-D442.html
In the firewire dept. I would probably choose
the Presonus Firebox, it has 4 analog inputs (2 with mic preamps, 2 line inputs), and comes with Cubase LE, it's a nice looking setup, and would be usable with a laptop if you have one, but at 400 dollars, it might be too pricey for what you want to do.
I am not a fan of USB-1 audio devices, I have
an Edirol UA-20 (2x2 I/O) and have not liked it, there's too much noise, maybe other ones would work better but I wouldn't recommend USB-1 at this time.
