It does fit.

The Razer Tomahawk ATX case allows for GPUs up to length of 384mm

Official specs RTX4090 has length of 304mm. Even biggest RTX4090 cards even go to 360mm. But check dimensions of the GPU you’re intending to buy of its length anyway 

PSU requirements - 850W min. 

GPU should come with adapters i.e. So it goes from 16 pin to 3x or 4x PCIe 8 pin connectors to PSU or if you have newer ATX 3.0 power supply which has newer just the one single 16 pin connector.

Yes it is fits with the Razer Tomahawk ATX.