In terms of an I/Q constellation diagram (i.e. a phasor or real/imaginary diagram), any frequency deviation from the center frequency would result in the IQ vector rotating around in a circle, with the speed of rotation being proportional to the frequency deviation. Essentially, the phase offset relative to your carrier/local oscillator would be continuously changing (because two waves of slightly different frequencies drift out of phase over time). The amplitude would be constant throughout this.
So, FSK would simply modulate the speed of the IQ vector's rotation. For 4-FSK, there would be four distinct speeds of rotation that you would observe over time.
This article has a good visualization of this near the bottom, under FM-modulation in IQ (this is essentially 2FSK):
Also, for a demonstration of why frequency offsets result in rotating phasors, check out this great video by w2aew.