i like to say yes. But Sounds more like internal. Have you take shifter off to see if thats the probrem? if T-45 is cable actuated trans like many import use, losing lower half of gear could be as simple as clip that hold cable comming lose(happened to my old MR-2. Lost 46 cents clip, and i lost 2nd,4th, and reverse, but 1st, 3rd, and OD was fine), or cable came off of shifter linkage.
But since T-45 shifter act directly to internal shifter fork, it doesn't have push&pull cable like import trans> I can't see shifter itself is the reason. If shift stop is causing this, that means shifter is not engaged all the way, there for shift stop have to "tigten" itself. I can see shift stop comming lose, but not tigten itself...
If your clutch cable is defective or stretched, i would think symptoms would also include 1st,3rd, and 5th gear as well...