You're comparing apples to oranges, somewhat.
On quick glance, the Yamaha MT4X is a true 4-buss mixer, giving it a true 4x4 (4-channel to 4-tracks simul) capability,... vs. the Tascam Porta Two, which gives you a 6x2 (2-buss) mixer with (switched) 4x4-simul capability. These are two distinctly different designs which are roughly the same in capability. Considering that, but then the Porta Two gives you 6 full channels, so on channel count in the mixer the Tascam wins the point.
Then, the Yamaha MT4X has 3-band EQ vs. the Porta Two's 2-band EQ on each channel, so this point goes to the Yamaha.
The Cue Monitor section on the Yamaha MT4x is slightly more complex than the Porta Two's, but given the two basically different mixer designs, it's almost a moot point. However, I'll give the Cue Monitor advantage to the Yamaha, if only slightly.
The Porta Two has real VU meters, and the Yamaha MT4X has a multifunction flourescent display. They both have their advantages and breaks out mainly on preference. The MT4x is obviously newer and more modern than the Porta Two, & is more on par with the 424mkII in some respects.
Battery operation and strap buttons for true field recording are features which go strictly to the Tascam.
There is RTZ, Memo & Auto-Punch on the MT4X, which the Porta Two does not have. Point: Yamaha.
They both have dbx NR, and they both run at double speed. Though I don't have a Tascam Porta Two manual right in front of me, I can see by the MT4X specs in the manual that the fidelity would be right on par with the Porta Two.
The Tascam has dbx Type II, and the Yamaha apparently has dbx Type I NR, but I think on this point you'd be splitting hairs.
The Yamaha MT4X specs are 40Hz~18kHz freq. resp. w/dbx OFF & 85db SNR with dbx on. That's the basic range of all Portastudios and clones, alike, and there's nothing in the specs stating clearly that
the Yamaha MT4X would be an "upgrade" from the Tascam Porta Two, based on fidelity alone. It often gets more down to features than sound. [IMO/YMMV].
When I dig out a few Tascam Portastudio manuals, I'll verify the numbers, but I'd dare say they should be roughly the same as the numbers from the Yamaha MT4X manual. The Tascam Portastudios may actually give slightly better numbers, which is something I'll verify by the book sometime soon. I don't have any Tascam manuals right here at my fingertips, but here's...
http://www2.yamaha.co.jp/manual/pdf/pa/english/recorders/MT4XE.pdf
and
http://www.yamaha.co.jp/manual/english/index.php
I'll toss in one last thought, that though a MT4x may not be an upgrade to a Porta Two HS, if you're thinking along those lines a Tascam 424mkII would be a clear upgrade that's superior to the MT4x. The MT4x is a weak 424mkII clone, [IMO]. The Porta Two HS is a pretty competitive machine in the Portastudio class, though it's more vintage than others. I think the Porta Two HS stands up well in comparison as being as good or better than the MT4X. Of course, if the price was ultra-low, I might pick up the MT4x for kicks, anyway.
