I believe that they do keep them for a certain period, but 4 years sounds a bit excessive to me, can you imagine how many text messages are sent every day and multiply that by 4 years, I don't think they would have the storage capacity for it to even be a possibility.
I know they keep voicemails for around 6 months for security reasons, it allows police to check allabys and stuff. And when you think about it, each text is less than 1kb easily, so having a server with terrabytes of storage could easily store a few years worth of texts. Im sure they keep non security raising texts for a few months at least.