Definitely no such thing as a 300mm f/1.4. Nikon used to make a 300mm f/2 which was absolutely huge, so a 300mm f/1.4 would be... impractical, shall we say. It's definitely confusion over the aperture being written as 1:4, ie an f/4 lens.
The lens you're talking about is probably the Sigma 300mm f/4 APO Tele Macro HSM. It says Macro but it's only 1:3 at minimum focus distance, not a 'true' macro. I don't know anything about it personally but you can read some user reviews here:
http://www.fredmiranda.com/reviews/showproduct.php?product=129