Do the Unity 5.x licences apply to Unity 4.x?

I’m developing a game using Unity 4.x. I understand that with the free version I am subject to the $100K revenue cap. If I subscribe to a pro licence (which seems to be concerned only with Unity 5.x) will I be able to sell a game made with Unity 4.x without a revenue cap? If not, how would I go about getting a pro licence for Unity 4.x?

Also, is it necessary to purchase a pro subscription before beginning to sell a game, or do I just need to upgrade to pro before I reach $100K?

Yes, a Unity 5.x license will work on any version before it. You should be able to sell your game without a pro subscription for now. unless you’re a AAA game developer and you are expecting to earn $100K quickly! just be sure to buy the pro subscription the instance you do reach that amount.