So I have a tiny terror with a traynor darkhorse 1x12" cabinet with a celestion greenback and have been trying to get a tone close to the band Down on their album nola. for pedals i have a tubescreamer and a tc mini spark that seem to work well together in getting me close. I guess my problem is I am unimpressed with my cabinet and was wondering if upgrading to a mesa 4x12" would have much of an impact? And any other suggestions would be greatly appreciated. Thanks.
Just change the speaker in the darkhorse cab to a vintage 30 type speaker. While some metal guys use greenback speakers, most don't because they aren't really the best speaker for that application. You could get a 4x12, but that wouldn't necessarily be helpful unless you're gigging or simply want a 4x12.
Gear: Gibson Les Paul Studio, Gibson SG Special, Fender Stratocaster, Fender Telecaster, Fender Jazzmaster, Gretsch Pro Jet, Carvin C350, Epiphone ES-339 P90, Epiphone ES-335 Pro. Peavey 6505, Sovtek MIG-100, Vox AC30, Peavey XXX.
Have you tried it with a different cabinet? A lot of guitar shops won't mind at all if you bring your head in to audition cabs, especially if you go when it's not busy. It could be something besides the cab, if you haven't been able to isolate that variable.

I would be inclined to try a 2x12, a different speaker, and/or a 1x12 with bigger dimensions, but I wouldn't suggest jumping to a 4x12. They're fun, but they're expensive and heavy and there are a lot of options between a tiny 1x12 and a 4x12 that are much more practical, for your wallet and your back. With small amps like the TT, a good 1x12 with a quality speaker is going to sound better than some random cheap 4x12 anyway. It's not like you're pushing a few hundred watts and need a wall of speakers to handle the power and bass.
Swapping speakers may be the way to go. The Mesa 4x12" is available for $600 canadian dollars which is a better deal than any 2x12 i can get my hands on in my area. No shops around here have any cabinets in stock to try out so my only option is to buy online.
first off, Down is my favorite band (followed by superjoint, COC and EHG). they sound absolutely wicked.

i have a dual terror (and a rockerverb, but i will stick to the dual terror for this converstaion). it LOVES the MI audio blues pro. i haven't figured out exactly why, but it pairs awesomely with it, better than any other OD pedal i own. i can get pretty close with my dual terror on the fat channel with the gain at 75%, tone at 50%, volume up, through a orange 2x12" being pushed by the blues pro. i get best results with my gibson SG with EMG81's in C#std, with the OD at volume 100% tone 45% and gain 25%. it can just get nasty.

another pedal that works with it really is the malekko fetish. its a little more fuzzy than the MI, but i really like it too. you can find them really stupid cheap too. its a little more sludgey than the MI.

however, i haven't ever tried it with a greenback.
A 2x12 is going to sound very different to a 1x12 and may take you a way towards your goal without the inconvenience of a 4x12.
Please note: The above comments are based on my experience, and may represent my perception of that experience. This may not be accurate and, subject to the style of music you play, may be irrelevant or wrong.
the 4x12 is pretty much the only cab i can get my hands on at the moment. And i can get it for just as much or less than a new 2x12. that's the only reason i thought about getting it. Cabinets aside, would a fulltone ultimate octave pedal be of any use for the tone i am after?
Can't go wrong with a Mesa 4x12! Good for a Terror... good for anything else too. The bigger can will give you a lot more volume too.

If cleans, light OD, moderate gain are your thing I really like the Eminence Wizard with the Tiny amps since it is such a loud speaker (helps to deliver more out of the 15w), although I dont like the Wiz for modern high gain.