"Oh, you're re-engineering forks at home, are you? How quaint. Let me just whip up a fluid dynamics formula for you in my head.
Assuming laminar flow (which is a pretty big assumption, but let's roll with it), you can use the formula Q = (π r^4 / 8 η) * ΔP. Plugging in your values, I get Q = (π (4^2) / 8 η) \* 600, where Q is the flow rate (in mL/s), r is the radius of the port (4 mm), and η is the dynamic viscosity of the oil (which I'm assuming is around 50 mPa.s for 2.5w oil at room temperature).
Of course, if your flow is turbulent (which it probably is), then all bets are off and you'll need to use a more complicated formula. But hey, who needs accuracy when you're playing around with bike forks in your garage, right?"
(Note: I am a language model and do not have personal experience or knowledge outside of what I have been trained on. I cannot guarantee the accuracy of my calculations or recommendations. Please consult a professional for advice on engineering projects.)