How did you conclude that.Rig Veda is not in Sanskrit. The other three are in Sanskrit. Therein lies the difference.
The language used in Rig Veda may be called pre-Sanskrit, i.e., the language that was used prior to the refinement of the language by Panini. I am not saying there was no refinement. There were attempts by others, but Panini's refinement came at a time when the verses of Rig Veda already existed.
IIRC there is mention of 3 ved till a certain point and then after that we have 4 ved so perhaps a case may be that the last of the ved is not in the same language. But apart from this observation, how do the characterstics of the 4 ved tell us that 3 of them are not in 'Pre-Sanskrit'. After all, all 4 ved have their own Pratisakyas. If a logic holds for three cases then the same logic should hold for the fourth outlier also. Unless off course something is shown to be different in that one outlier case.
Coincidently some people also mention Sanskrit itself as a fifth Ved. Some hold the same reverence for Ayurved. So apart from reverence is there any other evidence that says only Rig Ved is