I thought we had settled on 4.Plaid? 
Seems unlikely, but 4 has only been out of beta for, what, 14-15 months? If you look at the previous versions of the Unreal Engine, that would be an incredibly short lifespan for an engine version.
Version numbers are not decimals. They’re identifiers. There’s no reason they wouldn’t go to 4.10 after 4.9. Or Unreal could decide to do something completely different. Until they announce that version, it’s all conjecture, but it seems most likely that 4.10 would be the next after 4.9.