I would be worried if I only had a 4 foot fence. That sort of thing is fine if you've got a really large heavy breed of bird like a Jersey Giant etc, but most hybrids or pure breeds are alot smaller and lighter. Nb chickens are jungle fowl which roost in trees in the wild!! They may not be very elegant fliers but I've had chickens go over 6 ft fences before. They can also jump quite high and quite capable of bouncing onto a nestbox, then the roof and over a fence.
That's why I clip the flight feathers on one wing of each of my birds. Any new birds I do feather clipping before I put them down in the enclosure. I had 3 new hens go over 2 x said 6 ft fences before I'd clipped the wings! Never made that mistake again! Chased through 2 neighbours gardens!!
And anything they can use to jump onto I make sure is away from boundaries or has edging of chicken wire to prevent them getting up on top of it. My hens can jump 4-5 ft to reach the best blackberries! 4 new leghorns in the quarantine enclosure, each wing clipped when I got them home. This morning saw one hopped onto nestbox, ready to jump onto coop roof and eyeing up fences! Put layer of overhanging chicken wire on coop roof asap!!