How do I insert the program requested bitrate from YouTube Recommended Uploading Encoding Settings?
Since Mulvya's habit of answering questions in the comments ruins our answer rate statistic, let's put this into an answer °v°


  Which is 4 Mbps. Now OBS is asking for video bitrate, how do I convert 4 Mbps to bitrate OBS is asking for?


Bitrate is the general term for the amount of bits per second (1 bit = 1 unit of information). Mbps stands for Mega bit per second, which is the unit in which the bitrate is measured (not to be confused with MBps, which would stand for Mega Byte per second. One byte consists of eight bit). Both the audio and the video stream have a bitrate, which correlates directly (though not linearly, depending on the codec used) to their quality.

1 Mbps = 1000 kbps = 1.000.000 bit per second
4 Mbps = 4000 kbps = 4.000.000 bit per second



  Also, I presume I be recording in Stereo which is 384 kbps. However, the only bitrates available are: 64, 96. 128, 160, 192, 224, 256, 288 and 320. Mono is 128 but I hadn't seen anything higher than 320 for mp3 audio bitrate and I don't know if the value it's after is in kbps.


320 kbps is the highest audio bitrate available for MP3, as defined in the MP3 standard (MPEG-1 Audio Layer III). If you are still not satisfied with the resulting quality at that bitrate (which I find unlikely), use a better codec. For example, MP3's successor AAC is generally considered to yield better results than MP3 while using the same bitrate. But really, 320 kbps MP3 is all you need.


  My knowledge in bitrates are low and I just know 128-160 are average for mp3 and I don't know if this is correct that 320 is lossless or close to it.


128-160 kbps are acceptable if you use a variable bitrate (VBR). For a constant bitrate (CBR), I wouldn't go lower than 256 kbps.

But realistically, you can just use 320 kbps for everything and not worry about it. The thing is, if your video will take up 4 megabits or more per second, the added 320 kilobit per second for the audio won't add considerably to the overall file size. So really, there's no reason to go with a lower audio bitrate. 

Also, MP3 is by definition a lossy codec. However, whether any human will be able to hear a difference between 320 kbps and a lossless codec such as FLAC is questionable (I maintain they are not, but everyone is entitled to their own opinion).


  I've just been checking the YouTube Recommended Upload Encoding Settings to get the best quality without overkilling the end file size.


Don't pay too much attention to the recommended bitrates. In my opinion, those recommendations are way too low. The thing is, YouTube reencodes all videos you upload anyway. So if you encode your video at your end first, you have two lossy encoding cycles. If you use a low bitrate, your video will contain artifacts after the first encoding cycle already. Those will only be made worse in the second cycle. So if you use a higher bitrate for the encoding on your end, the endresult on YouTube will also look better. I would only go for a lower bitrate if you have a metered internet connection or your connection is so slow that your video uploads take an annoyingly long time.