How far is it safe to run the stock Pirelli?

[Husafreak]

I have 4000 miles on my stock Pirelli Scorpion rear tire (already changed the front to a Dunlop Trailmax Mission). Smallest read depth is 4mm and the tire is noticeably squared off in profile. I've ridden 90% pavement so far. I want to go on an approximately 1800 mile road ride to Idaho next month. I would like to do the ride with this tire but I'm concerned that will leave me riding home through Tahoe on a really worn out tire. I have very little experience with road riding, just this bike. If this was a dirt bike knobby I would change it if racing and run it if play riding. But on the road how far can I safely go? To me it is a wierd looking tire without a center tread, just those grooves across the tire. We will be doing some aggressive riding on back roads and twisty stuff, not just highway. Anyway, maybe some of you with a lot of experience can chime in. Thanks

[AZJW]

@Husafreak 4mm is not sufficient to make a trip of that length imho.  Safe bet is to change it out as in the past I've had supposedly decent tread left to make a short little 300 mile ride and had cord showing at mile 200.  

[Husafreak]

Thank You!