In my experience,
GC4 pros-
#Already has todays K55 profile cushions so its pretty safe in swapping cushions without rail modification if you need too.
#Flushed Pockets unlike the GC1 through GC3, cleaner looking.
#Aprons are bolted on.
#I like the GC4s overall look compared to the 1s 2s and 3s. Although a fully restored GC1 with the skirts looks amazing.
Cons-
# Frame likes to sag on the ends. Need to be braced up. I don't know why Brunswick changed the design of the frame but angle iron is a quick fix
#Levelling system is not great, I prefer to still use shims.
GC1s/GC2s/ and early GC3s
*Rails need to be calibrated for todays rubber, meaning k55 nor k66 will swap for the original monarch superspeeds . Only a handful of people can do this work. The last statement (*) is only if you want the table to play its absolute best. There are a lot of different opinions on how a table should play.
The frames on the 1s and 2s seem to be a lot more solid than the 4.
The GC1s and 2s have a better corner casting bolting system in my opinion.
I am sure I am missing some other things.
