TK is right - that car belongs (belonged?) to Phil from Vette Connection. dd on this forum and myself also have fixed back race seats in our C4's, we all run Sparcos.
I don't know if you have tried Sparcos but I know they make a few seats that fit in the "tub" of a C4. I took measurements many times before I bought my seats and it was a very close fit - but they fit.
Sparco make different size seats - I am running "Corsas" as they have the harness holes high enough for a tall person like myself, the evo is for shorter people and the evo 2 is a little wider in the base, there are a number of other models but Corsas, Evo's (both models) and Pro 2000's should all fit based on the homework I did when I bought mine. You can download the dimensions of the seats from sparcos website and do your measurements there but I would strongly recommend test sitting in a seat before you buy.
I'll post links of pics of my seats shortly.
If you have any specific questions let me know.
What brands did you try out?
Cobra may make something as well but I never really looked too hard at what they had to offer.
Hope this helps.
BTW, you think your C4 is hard to get out of now - wait till you get race seats... ;41