My b12 levels were slightly under 300 and my doctor diagnosed me with pernicious anemia, which is when you don't absorb b12 through the stomach. I'm on shots every 2 weeks and will have to continue them for the rest of my life; and yes, it does help me feel better. I'm also hypo and am on generic levo and cytomel.
Make sure you get your vitamin D levels checked after a while on the 4000 IU as vit D can be toxic if your levels go too high.
I also had low B12 (250 the last time it was checked) and what my dr. told me was that people with low B12 do not absorb it normally which is why it is low. For this reason, I would have to get B12 shots. I will say, I do feel better with the shots, and my dr. wrote me a prescription for a year with the needles so I give myself the shots once a month now.
Good luck, I hope this helps!