The only one that I am absoulutly certain of the the 61X (LaserJet 4100) is backwards compatible with the 27X (LaserJet 4000/4050). We only stock and sell the 61X as they will work in 4000, 4050, and 4100. The 27X's however will only work in the 4000, & 4050.
And you may have noticed the A and X designation on some cartrides, like 27A and 27X. These are the same cartridges, but the X is a high yield, and offers more prints.
Timothy N. Couch
A1 Printech
Anchorage, AK