Hmmm... the only gap i see is better spread at slightly lower heights..... and slightly higher par. Is that enough to justify 20% for each light plus the given fact the g4 XR30 are being sold with diffusers? It's all moot at this point as @piranhaman00 said they are sold out. I currently have 3 G4 XR30's on my current tank,hower i have moved to Orpheks for the new build.... i think overtime to address the directional nature of LED's more manufactures will move toward panel style lights... the new layout of the G5 by ecotech is a signal to that direction anyway.I wouldn't waste money on the G4's. I'd personally put in the extra money and get the G5's.
As a long time radion user, I feel the Gap is too large between the G4 and G5. I wouldn't purchase a G4 just to save a couple hundred. This isn't like a G3 to G4 where we have a minor upgrade.