I use a PR4 as the personal item with a SkyTrain. I have never had an issue fitting it under the seat and it has never raised any questions at the gate either. It's a perfect carry-on personal item.

Interestingly, I have a FarPoint 40 like you, but have not used the PR4 with it. Rather, I've used the XS Aviator, probably because it and the Farpoint are both black. (PR4 and SkyTrain are both safari)