The 134a runs at higher pressures.... and there are plenty of people who have had short lives out of their compressors after changing to it...
and on the other side... many people with 20 years use out of their compressors using R12 ( which was designed especially for your AC system).
I don't know if it is the pressures by themselves... or if the oil used with the 134a does not move around as well, or lubricate as well ... but compressors used to last a long time with R12 in them.. and cool better...
The oil which travels around is supposed to mix with the refrigerant... miscible ( spelling )... so maybe that does not do as well as the combo of mineral oil and R12...

you are correct about the R4 not having a crankcase and being more sensitive to lube problems... I will have to research about the oil amount which is supposed to be in the system with regards to the upper end of the York.... your's is an OLD system... LOL