As a fairly recent chicken-keeper, I had/ have a similar concern with Barney our Springer Spaniel, so I clipped the wings (one each!) on our hens shortly after getting them. However, Nutmeg (our birds are all named after spices!) still occasionally clears their 4 foot fence with what I call her W.A.L.L. (Wing-Assisted Leap and Land) technique! After she got out a few times I watched from a distance and observed her combination of a jump coupled with a few wing flaps, which is enough to take her over the fence without having been knocked too far off balance (which would obviously happen if she tried flying further).
Unfortunately, it seems the others are starting to learn this behaviour and it did lead to an "interesting" (!) moment last week when Pepper came over the fence as I approached (she'd spotted the tub of dried mealworms I was carrying) and, before I could react, Barney rushed past me and pounced on her. Fortunately he just held her and did not bite, so I was able to pull him off and haul him away with a scolding. Thankfully, Pepper was unhurt and back feeding etc as normal within a few minutes, although went off lay for a week (just started again yesterday).
Yesterday I flattened a slight earth mound in the enclosure which Nutmeg had been favouring as a take-off point and I'm hoping that will discourage her/ them. I'm also continuing to train Barney - over about six weeks he's gone from initially barking, and trying to rush the fence, to generally ignoring the hens - but the sight of a flapping, clucking, chicken landing just in front of him was clearly more than generations of instinct could stand!
Happy hen-keeping! ;-)