Hi Flip,
The level of the signal from a guitar pickup is somewhat higher than that of a microphone, but as long as you start with the 'Mic Gain' control on the keyboard set to 'Min', and very slowly and carefully increase it while playing the guitar, you should be able to at least set a gain level which doesn't distort.
However, the output from a guitar pickup is very high impedance, much higher than a microphone, and the input impedance of a microphone input is optimised for that of a mic. If you connect the guitar to the mic input, it will change the sound somewhat, typically with a loss of high frequencies. You can decide whether the sound is acceptable to you, assuming that you can actually hear any difference - my high frequency hearing is rubbish so I probably wouldn't notice!
If you can set an acceptable level, remember that you can try using the settings on the 'Mic Settings' display. There is '3 Band Equaliser' on this display which you could try to modify the sound using this if necessary!
The relevant section of the Tyros4 manuals for Mic Settings start on page 94 of the 'Owner's Manual', and page 118 of the 'Reference Manual'.
Regards
Ian