ITPS is crap. it's only a linear regulator, so it requires 13.5v input to produce 12v output; anything lower and the output won't stay at 12v. You can buy a separate linear regulator for like $5 if you don't need the logic, but the input voltage range is just worthless... only really even works if the car is running.
Have you seen my DC-DC converter design? My weekend project...
it outputs ONLY 12v so it would probably work well for your application... and I think it would be fine at 4 amps output. I don't know the input voltage range yet, but I do know that it survives engine crank in my car... And I've put less than $20 into it.
On another note, might I ask why you don't think the 8-19v input tolerance of the Opus is good enough? 8v is enough to survive crank, which is about all that matters on the low end, and 19v is quite high, definitely higher than you'd ever see continuously in the car (not counting short spikes, which would generally be decently absorbed by caps and inductors in a supply anyway) I can't see any reason to need more than that on either end. After all, the more specific you can make something, the more effective you can make it... making a DC-DC work with a gigantic input voltage range would probably mean it wouldn't be as efficient... there's tradeoffs for everything.