It seems G4 only defines the 4mm pitch of the pins and for preference in different counties there were 12Vac and 24Vac type halogens.
For compatibility now G4 LEDs are made to adapt to one or the other.
They all tend to use 4 discrete Schottky diodes must be labeled with the Vdc rating.
For 24Vac = 34Vp , and if using 8 in series would be OFF @ 20 Vdc, DIM @ 23Vdc , then with a choice series R in the 30-40 Ohm range would MAX @31Vdc. With a transformer rated for Halogen bulbs, the voltage would be higher than 24Vac perhaps 26V which would fit well with the RS value and 8 LEDs
For 12 Vac = 17Vp, they might use 3 LED stings with 75 Ohms and the same 4 diodes
If 4 LEDs with diodes the series R would be smaller about half of the ESR or 5 Ohms.
If 9 LEDs@12Vac/dc They would be 3S3P with string Rs values in the range of 75 Ohms with diodes.
My hunch is that the reason for failures is PLT or power line transients not bad LEDs.
All Schottky diodes with lower forward voltage (lower ESR) also have lower PIV rating from the same style, vendor & size. e.g. 20,30,40V.... due to gap length.
But the luminaires may not have MOV's. This is a customer power quality issue.
But there are many many variations of G4 LEDs.
Next is 230Vac
next is 12Vdc only with a simpler Switching regulator
http://miniimg4.rightinthebox.com/images//201303/tuazya1362129984549.jpg
http://miniimg3.rightinthebox.com/images//201303/drrgmn1362129984085.jpg