The short answer is, it should be .5" shorter than the 4h. But will it make it your 190 club?
Before you do anything, take it to the range with yardages you trust, a simulator, or a local shop with a launch monitor. The thing is, you're sitting on two clubs that are, basically, the same club. The reason why a 6i is shorter than the 5i isn't just the length, but the loft, as well. Remember the ".5" " thing? Just doing that won't be enough to cut 10-20y off the overall distance, because that likely won't be much of a change. Could you cut, say, 1.5" off and turn it into the 190y club? Maybe... but then, would that make it shorter than the 6i? What about what happens to the overall playability of the club (swingweight too light, altering the shaft flex, etc.)? All that said, it's still possible to alter the club a little and have it be the 190club... and that's why you go to the range (etc.). Grip down on the club .5" and hit some shots. Do it again at 1", 1.5", etc. Each time, keep track of how far the ball's going. You'll also likely notice the change to the club's feel, as well; if you find the length that nets you 190y, but you don't like the way the club feels, would that be a trade-off you're willing to make?
Why not take the "inaccurate" 5h to a local fitter? Take at least the club before it (the 4h) and the club after it (the 6i), as well, so the fitter can "diagnose" what's wrong with the 5h. It could be some specs are off, like a shaft that's too light (and/or the wrong flex for your swing), it's too long (miss the "sweet spot" too often), or the swing weight's too light/heavy for what you need.