ibanez soundgear bass truss rod tool?

[ejackson92]

I recently bought a 4 string ibanez bass, I know the model was made in the early 90s because of the serial number. Strangely the model number is absent from the back of the headstock. It is clearly identifiable as an Sr series however.
Nevertheless, I need to adjust the truss rod but am unsure what kind of tool and what size to purchase.
Help out a newbie!

[Solarmist]

4mm hex on all mine. Ibanez SR4 usually requires very small adjustments unless something is way off.