this file is not a movie file mp4

So I am wondering... can I download something to help me open mp4s?

Maybe, but you may have to determine what is wrong first before knowing what to look for to download.

I thought I could open mp4s on a Mac...

MP4 containers encoded with standards-based Progran Stream MPEG-2 or H.264 video and AAC audio employing supported Part/Profile/Level settings are natively QT compatible.

The conversion company however has given me an MP4 which I am sure has some sort of compression on it (when I wanted it uncompressed, e.g. as a QT uncompressed).

While some multiplexing utilities have the ability mux non-MPEG-4 content to an MP4 file, QT-based apps will not play such files. This does, however, bring up another question—if you dulplicate the file at the Finder level and you manually change the extension to MOV, will any of your apps open the file?

I am trying to open a 10Gb mp4 file... Is it over some sort of duration or size limit?

This might be a problem for QT-based apps (QT 7/X players, GarageBand, iTunes, iMovie, FCP, Compressor, MPEG Streamclip, etc.) since "long addressing" is required for files greater than 4 GBs. (Not sure if this is a problem for transport stream media players like VLC.)

The footage also appears to have no duration. (??) Even though the size always shows up.

This could also be a problem of "short addressing" of a file over 4 GBs. Since QT-based apps cannot "see" the end of the file (EOF), it can't calculate the timing offset (duration) of the file from the "base" time at the beginning of the file. (I.e., while most files start with a time of zero, they don't have to do so.)

Or is the file possibly corrupted? Having no duration an not being able to import or open it in any way seems to tell me something screwed up in a file conversion process.

Corruption is always a possibility. However, I would normally not expect it to be introduced if the content was professionally processed. A 10 GB file for 90 minutes implies a total combined Audio + video data rate on the order of 14.8 Mbps which sounds about right for a reasonable quality H.264 HD video encoded file using any of the common QT supported Profile/Level combinations.

MP4 containers encoded with standards-based Progran Stream MPEG-2 or H.264 video and AAC audio employing supported Part/Profile/Level settings are natively QT compatible.

Oops! Should ha read "MPEG-4 or H.264 video" above.