You do not have a logic-level FET. The Vgs rating at 2V-4V says that if you ONLY need to supply 250uA through the FET (from source to drain) then you won't need any more than -4V on the gate-to-source junction. Beyond that, they don't tell you anything until you have -10V gate-to-source, at which point you could source 13.5A and have an on-resistance of 0.07 ohms.
In between...you just don't know (well, you can predict and model, but there are no guarantees). The reason you are using it "without problems" is likely because you do not source a lot of current through the FET, thus your -5V gate voltage is good enough for the job, but I really doubt the FET's on resistance (if you measure it) will be near 0.07 ohms.
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