I would not refer to it so much as 'nice and tight'. The bolts are shouldered meaning they go down to hit solid and then anything more you put on you are simply twisting the bolt in two and pulling threads too. I simply go down evenly until I feel them hit and then just enough more to verify they did hit. Using torque wrench there is quite frankly irrelevant since the hit tells you to stop.
6 mm.-1.0 thread pitch like these typically strip aluminum threads at anything over like 9 ft.lbs. I'd stay at 4-6 but if using ft.lbs. wrench you probably have one that is too big for the job, what's needed is a short 6 inch one with a torque that low. The bigger one is simply too big to be accurate with torque that low.
Doesn't matter anyway, I never torque them. Once you figure out all they can do is hit and no further gasket squeezing can occur after that, it becomes easy to simply feel it hit and pretty much stop there. Never broke a bolt or pulled threads, and never have leaks either.
Same thing with the thermostat housings on these, the bolts hit to limit of part, no tightening you can do after that helps a thing, all you are doing is breaking or cracking your stat housing, the damage then shows up later because you preloaded the plastic part too much. The parts were not meant to be that tight, once you hit part solid you have compressed all the gasket that is possible anyway. I love it when guys here talk about going back to retighten more when part leaks, I can practically hear the part crack. Look at how many people have to buy new housings here, I've got 3 zetecs back to '98 and have had housings off all 3 many times and never failed one yet. I don't torque these either, you again look for the part hitting solid, no torque wrench needed.