Reading the documentation on Lightmass (which is supposed to be fully GPU supported in 4.26??) and confused by the reference to UE 4.9??
CPU Lightmass Global Illumination in Unreal Engine | Unreal Engine 5.0 Documentation What exactly is UE4.9?
It’s not almost UE5, sadly 
It’s a very old version of UE4 instead.
UE4 4.9 is really UE4 4.09… with 4.26 being the most recent release (soonTM).
The numbering is like that.