I have to change the bitrate of MP4 files on Mac OS X. What's the best tool to do this conversion? Am I able to do it with QuickTime?
You can use any video encoding tool to change the bitrate of an existing video. The choice is yours – and on Mac OS X, you probably want to use one of these:
First things first: Re-encoding an already existing file will reduce its quality! This is called Generation Loss and will affect any lossy material. Only re-encode the video if you really have to.
Encoding to another bitrate with Handbrake
Let's use Handbrake, since it's cross-platform and free and easy to use. Open up Handbrake, open the input video, then look at the Video settings.
Set the average bitrate to whatever bitrate you want to use. With h.264 codecs—depending on the resolution of the input video—you can choose anything between 500 and 2000 kBit/s, or even more. The setting will heavily influence the quality though!
Intermission: Do you really want to encode with average bitrate?
Also note that changing the bitrate to a fixed rate is not always beneficial for quality. Actually, if you want better quality for a certain file size, choose the Constant Quality setting instead of Average Bitrate, and use an RF (Rate Factor) from 19 to 24.
Here, you'll have to experiment what looks best, but setting a CRF will most likely give you a better quality. See this guide if you want to know more about how the CRF works.
So, unless your scenario is video streaming, where you really need a fixed bitrate, you should probably use the CRF method instead.
It's definitely possible to re-encode a video with QuickTime. First of all, QuickTime Player makes your options rather limited. Open the video, go to File » Save As…, and tick the versions you want to export, e.g. "iPhone", which will give you around 1 Mbit/s bitrate.
If you have QuickTime Player 7, you have more options. Go to File » Export, then choose Settings… under Video.
Here, choose the H.264 codec, and restrict the Data Rate to whatever you want.