Director Tools is an Unreal Engine 4 plugin which works seamlessly with Editor. Although several timeline editors exist for Unity, this is the first 3rd-party timeline for UE4 and it looks great. Features include image sequence export. Licenses are around $50 and $200.
[relatedpost limit=5 position=none thumbwidth=240 thumbheight=140 size=20 color=#666666 hovercolor=#FFFFFF]