You asked
Sony fe 400mm f 2 8 gm oss vs sony fe 300mm f 2 8 gm oss?
Both are Sony E-mount prime lenses for full-frame cameras. Both are f/2.8, stabilised, and autofocus, and both fit the same long list of Sony bodies, full frame and APS-C alike. The main difference is reach.
The Sony FE 300mm f/2.8 GM OSS frames as 300mm on full frame, or about 450mm on APS-C. The Sony FE 400mm f/2.8 GM OSS gives 400mm, or about 600mm on APS-C.
My take: if you shoot birds or distant animals and want every bit of reach, get the 400mm. If 400mm is more than you need, the 300mm gives you the same bright f/2.8 with less length. For planning reach, see Wildlife and sport.
Written by Claude from this site's pages.