I see so many laughably wrong power level multipliers, like that one guy who just claimed SSB is 1 million times base power, so I'm going to put here what is actually known or can be reasonably ascertained based on the series' lore. I'll only be listing proper Super Saiyan transformations (no "False Super Saiyan", no power stressed/weighted buffs, etc.), with the exception of the Great Ape, since those are the ones most people care about, and the last one is relevant to SS4.
Dragon Ball & Dragon Ball ZEdit
Great Ape: 10x base power
Super Saiyan 1: 5x Great Ape = 50x base power
Super Saiyan 2: 2x Super Saiyan 1 = 10x Great Ape = 100x base power
Super Saiyan 3: 4x Super Saiyan 2 = 8x Super Saiyan 1 = 40x Great Ape = 400x base power
Dragon Ball GTEdit
Golden Great Ape: 50x Great Ape = 500x base power
Super Saiyan 4: Golden Great Ape = 1.25x Super Saiyan 3 = 5x Super Saiyan 2 = 10x Super Saiyan 1 = 50x Great Ape = 500x base power
Dragon Ball SuperEdit
Super Saiyan God: >400x base power
Super Saiyan Blue: 50x Super Saiyan God = >20000x base power
X10 Super Saiyan Blue Kaio-ken: 10x Super Saiyan Blue = 500x Super Saiyan God = >200000x base power
X20 Super Saiyan Blue Kaio-ken: 2x X10 Super Saiyan Blue Kaio-ken = 20x Super Saiyan Blue = 1000x Super Saiyan God = >400000x base power
What this means is that, at the very least, Super Saiyan Blue is more powerful than SS4, regardless of SSG's multiplier, since SSG already buffs base power by an unknown amount that is higher than SS3's multiplier of 400.
Enjoy these accurate multipliers.