The GPIO on the Pi4 is 3.3V.
You will need some external circuitry but without more details it is impossible to suggest what.
It is simple to convert voltage levels - the complexity depends on speed requirements and whether unidirectional or bidirectional is needed.
You indicated "hints" were OK, so here's a hint-but-not-an-answer:
I suspect that there is a way to get 1.8V out of the RPi 4B - but this is only a suspicion. I only read of it a day or two ago, and I've not tried it, nor explored the idea in any detail. The basis for my suspicion may be found in the CM4 datasheet. Reference para 3.3 Electrical ...
According to the following document (Fig. 5) the 5V sensor can be directly connected to 3.3V Raspberry Pi SPI.
However, still struggling to get it work, with no success. I have captured the SPI waveforms and are attached.
Any further help will be appreciated.