Well playing around with curves > character animation in app can get messy but you can create the stop motion effect by changing the key type in the host app from linear to step.
The step key will hold the pose until the next key frame but depending on the host app you will need to bake out the per frame animation to FBX so that it will work and play back in UE4 with out interpolation and you don’t have to mess around with curves as the animation data set is absolute.