In OS X Apple (for one reason or another) artificially limits the maximum RSA key size for certificates to 4k (4098 bits). Most users will never notice any issues since most sites use 4k RSA keys or lower. However – for the handful of people who use/own websites that use 8/16/32 etc bit RSA keys, this artificial limit can cause problems. Below I’ll run though how to change the key size limit with 1 quick line in Terminal. As ever, please ensure you’ve read and agree to the disclaimer (linked here) before continuing.
At the time of writing Apple have artificially limited the maximum RSA key size on OS X (and perhaps iOS too) to 4098 bits.
Side note: the way you change the limit differs between OS X releases.