There was an Up loop which pretty much mirrored the formation of the Down. This was removed in the early 70s when Trumpington signal box closed. Never 4 tracks under the bridge.
But perhaps it was originally built like that with the possibility of 4 tracks in mind, otherwise it seems an unecessarily long and expensive structure, ditto the wide formation.
A bit like Letchworth station which AIUI was intended to accomodate 4 platform faces, hence the false "islands" https://en.wikipedia.org/wiki/Letchworth_Garden_City_railway_station.