Why does my c4 transmission go into reverse only

Your C4 transmission works by using something called planetary gears. They're really a fascinating piece of engineering, as you can get different speeds and even reverse the direction of rotation depending on which part of the gear mechanism you direct power to and which part of it you hold in place. That's the function of the bands and clutches inside the transmission; the bands wrap around the outside of the planetary gear assembly and keep it from turning, while the clutches hold parts inside to either couple them to the input shaft or the output shaft.

The C4 has separate bands for forward and reverse gears, so it's quite likely that the intermediate (forward) band isn't working, while the reverse band is fine.

All of those bands and clutches work by hydraulic pressure, so besides the possibility that the band itself is worn out (just like brakes or the clutch on a manual transmission), or it could be that the intermediate band isn't getting any or enough hydraulic pressure to activate it.

You really need a mechanic experienced in automatic transmissions to find out exactly what's wrong with it, but it's usually not something home mechanics can take on themselves. Replacing a valve body or a modulator is doable, but first you have to find out exactly what's wrong to figure out if it's something you can fix yourself. Many transmission shops will give you a diagnosis (usually for a fee) and, depending on what information you get, you can then decide if it's a repair you can do yourself or if it's time to hand it over to a professional, or perhaps consider replacement.