i tried a 4stroke heli about 4 years ago as i fancied a change from all my 2 strokers.

i brought a good condition vario sky fox cub which already had the conversion kit in it and the os 91 4 stroke which also was good condition and lots of compression.

i was though very disappointed with the power as it only just was able to hover and gradually i could creep foreword and back but that was about it..
i tried a couple of other things like changing the blade length but though it improved a bit it didn't make it very flyable as it still was barely just able to hover. that blade length had to be reduced though to a more or less unacceptable amount as the heli is mostly flown with 68cm blades with a 2 stroke but it wouldn't lift off the ground unless i used 62 cms or less..

if you do a search on this forum though there is a heli flyer that had good experiences with 4 strokes on helis but sadly for me by the time i read his advice i had already put the 2 stroke engine and kit in the heli.
i think he mostly advised about 30% nitro fuel and the type of exhaust might be very important as well.

hope this helps