If that was me I'd fill the tank up and see what happens and keep an eye on it, the reason I say this is because my fuel gauge moves about quite a lot when it's running low, as in, one minute it's on a 1/4, then it's over, then if I go round a corner or up an hill it then goes into the red etc etc. The only real way to check how much fuel there is in your tank is to see what the gauge reads on the flat, either with the engine running or with the ignition on. The upshot of this is that it may move slightly when it's running around the 1/4 tank mark, most cars do anyway.
