Of course it will just fit in (thanks to ATX-standards), because the 8-Pin EPS-variant on your motherboard is just 2x 4-Pin (but mirrored). Using only one 4-Pin connector halves the amount of +12V and GND pins to two (2) Despite your PSU featuring only single rail +12V, it really won't make that much difference if you use the 4-Pin connector only. The Core i7 8700k has a TDP of 95W (according to Intel) which the two +12V cables a 4-Pin connector features can handle. If you take a total of ~4 Amps per line (95W / (12V x 2), that should work. Suggest to just test if the PC will boot at all and than do a stress test of > 30 Min while you watch for smoke etc.

4 pin cpu power on AnandTech
PSU connectors --> scroll to shortly above 8Pin EPS
Image of 4-Pin and 8-Pin (EPS) connector pin out

Of course it will just fit in (thanks to ATX-standards), because the 8-Pin EPS-variant on your motherboard is just 2x 4-Pin (but mirrored). 
Using only one 4-Pin connector halves the amount of +12V and GND pins to two (Instead four) Despite your PSU featuring only single rail +12V, it really won't make that much difference if you use the 4-Pin connector only. The Core i7 8700k has a TDP of 95W (according to Intel) which the two +12V cables a 4-Pin connector features can handle. If you take a total of ~4 Amps per line (95W / (12V x 2)), that should work. Suggest to just test if the PC will boot at all and than do a stress test of > 30 Min while you watch for smoke etc.

4 pin cpu power on AnandTech
PSU connectors --> scroll to shortly above 8Pin EPS
Image of 4-Pin and 8-Pin (EPS) connector pin out