RSA private key format for Mega
The format is not an official standard, but it is pretty straight forward. There are indeed 4 fields, in order $p$, $q$, $d$ and $u$ (I'm not quite sure what the function of the $u$ is?, it's not the CRT representation that is common in standards). 

First decode the base64: it starts with 0x04 0x00 0x8b 0x91 0xff 0x6a 0x8f 0x5c..

Every number is encoded as a big-endian 2 byte number that denotes the number of bits
(So here 0x400 = 1024), so we then know the number of bytes (so here 0x80 bytes) that then follows and this is just a bigendian number ($p$). After these bytes we get the second length field (0x400 again) followed by 0x80 bytes (for $q$), a length 0x7fe = 2046 bits so 0x100 bytes that encode $d$ (it makes sense that this should be roughly twice the number of bits of $p$ and $q$, as it is of order $(p-1)(q-1)$) and the last $u$ is again 0x400 = 1024 bits, and after this number we get 0x2d = '-' characters as padding.

Normally these private keys are ASN.1-encoded (e.g. PEM format), and Microsoft has its own format in Windows, etc. Many such encodings are just a decorated version of the length followed by data of that length format, so this is pretty easy to figure out.

Take a bignum library (I use GMP, normally) and try to convert the bytes to numbers and check primality etc etc. to try to verify this.