Right you will read many that have problems with running engines inverted, both 4s as well as 2s. I have both in several planes and do not have any problems with my engines when inverted or sidewise or upright. Ok, will modify that slightly from time to time there have been some that become troublesome but rarely and the problem is not due to how they are mounted. Just something inherent with model engines. sometimes they just become cantankerous. Main things are proper tank location and engine tuning. These are both well covered in other posts. Of course if you go electric none of these problems exist.