I have seen some Osram high pressure sodium lamps marked "Vialox" and some are marked "Vialox 4Y". Is it just a newer version? A slight change to the construction perhaps?

In some way they increased the life rating to 24+khours and a mortality curve high enough, which allows 4 years replacement interval (so after about 15k hours) in group relamping schedules while still meeting the maximum 2% failure rate criteria.

Dunno if that meant a real technical lamp design change, or whether just the data collected from the field had shown the lamps already did perform that way.

I see so 4Y = 4 years, makes sense! I've just ordered one of each. I couldn't see a difference in the photos of the lamps. Will be interesting to see if there's any perceptible difference viewing them in person.

The lamps are here and I have posted a comparison photo in my gallery! Turns out the internal construction is definitely different, especially the ignition wire.

As far as I know, the Osram NAV 4Y high pressure sodium lamps have a much higher xenon pressure compared to regular Osram NAV high pressure sodium lamps. In addition, the 4Y lamps have a higher luminous efficacy compared to standard lamps.