I also use the Polar Pro ND filters with my X4S (and X5S as well although they are of a different size) and find them really good. The colour tone is pretty neutral compared to some other filters I've seen (but not tried myself). I'd go for quality over price and would recommend Polar Pro. As you're using the filters with the X4S pay attention to which kit you buy from Polar Pro. They have a VIVID and SHUTTER kit - the former are ND/Polarising filters whereas the latter are straight ND filters. I had the ND/Polarising filters but found that if you set the filter for proper polarisation whilst on the ground then once you get up in the air you have to make sure you aim the camera just right with respect to the sun so that the polarisation still works. This was a lot of hassle and I bought the SHUTTER kit instead.