I am relatively new to multitrack recording. Apart from having recorded a few pieces on my iMac, I have been recently transferring to my Mac some recordings I did 10 years ago on my 4 track portastudio for the purpose of 're-mastering'. I was struck by the relative richness of these old recordings to my more recent ones and was wondering whether I stumbled into the old analog vs digital problem. If so, does a product like T-Rack-S solve the problem or do I simply need to learn how to better use the EQ and VST plug-ins on my multitrack software?

Getting a fat sound with digital is not that hard:
First be sure that your A/D converter are goods. Without serious converters, any effort you spend before the converters, would just be a waste of time (and money).
Then use the correct preamp to boost your low-level (often low-Z,) source (mic, guitar, synth, ...) to the correct level of your converter input.
Always remeber, that you MUST always record a HIGH level in your DAW, if you want to get a sound at least as good as a tape recorder: Due to the quantization process, if you record a sound too low, you will be faced with the quantization noise that is really ugglier than the noise produced by tapes recorded at low levels.
With a good preamp, a good converter, and A GOOD LEVEL, you should get a really better quality than a 4tr. tape.
To add the warmness you like on tape records, good tube preamps work fine!
My favorite include Tubetech (or Pultech) and Summit ones, but they R a bit expensives