How does the lewis structure for XeSe4 look like?

There's a total of 32 valence electrons so you will have Xe as the central atom with four Se atoms attached. Each Se will have 3 lone pairs surrounding them. This results in an electron geometry that is tetrahedral (sp3) and a molecular geometry that is tetrahedral

The compound XeSe₄ does not exist. Xe can only form compounds and F and O.

For the hypothetic compound XeSe4 (assuming that it exists), its structure likes XeO4 as both O and Se belong to Group 16 (or Group VIA). The structure of the hypothetic compound XeSe₄ is shown below.

https://c1.staticflickr.com/5/4854/46175652701_3569b3abcd_o.png