Despite reading about this same issue that date back over 3 years, I have yet to see any resolution so I will post my issue anyway.

My system:

Windows 7 Home Premium 64 bit SP1

WLMM version 2012 (Build 16.4.3528.0331)

Source file:

Recorded with nvidia Shadow Play with the following settings:

Res: 1920x1080

FPS: 60

Bitrate: 22000kbps

output file: .mp4

WLMM custom settings 

Res: 1920x1080

FPS: 60

Bitrate: 22000kbps

Resulting file after WLMM renders the video to .mp4

Res: 1920x1080

FPS: 60

Bitrate: 11869kbps

WLMM custom settings (new attempt)

Res: 1920x1080

FPS: 60

Bitrate: 50000kbps

Resulting file after WLMM renders the video to .mp4

Res: 1920x1080

FPS: 60

Bitrate: 11869kbps (obviously no change)

When rendering to .WMV however, the bitrate is preserved however the quality appears to be very degraded despite the same bitrate.  I would like to save to mp4 and preserve the source file as is with no re-encoding by WLMM as it simply does not function well (after years from what I see here and zero change).  Is there any way to do that??  All I want from WLMM is to string several .mp4's together, add some pictures but I don't want WLMM do anything else.  I do not want to save as WMV

Please help