Alright, here's my logic behind the claim.
>SS4 isn't even really what one would call a super saiyan form, in the sense of the last three. those are multipliers and stamina drainers, thus you feed your body stamina, and it multiplies your ki output.
>SS4 is really a form shift, not unlike Freiza's transformations(before golden), and other physical metamorphoses. It doesn't require stamina to maintain the form, but ki (If you run out, that is when it turns off)
>SSG forms [Red and blue for brevity] Require MORE stamina to stay in, and seem to be traditional multipliers, but are somewhat more flexible in ki output than Regular super saiyan forms. Rather it seems it takes more stamina to transform into the forms, but the output of the stamina is greater, and FAR easier to control, with none of the restlessness. Also Allows for kaioken.
>SS4 However is a limit breaker. What does that mean though? How is it significant? Well, lets see what else is a limit breaker... Ultimate/Mystic Gohan is a limit breaker. So, SS4 is best understood not as a 10x multiplier to ss3 (that would be retarded) But as a transformation similar in vein to what gohan went through. That is his potential being completely unlocked, and ss4 is the same.
A couple of things that make ss4 "better"
-Duration of transformation is WAY longer.
-Power level can be suppressed with form maintained
-Staying in it doesn't drain stamina
-Highest ceiling for Ki is unlocked
-SS4 has it's own "saiya power/Zenkai" that work while in that form, but don't power up base. That is what makes it a limit breaker, rather than just the limit.
-Since it is a truly physical metamorphosis, being unconscious or low on stamina/ki doesn't knock you out of form. One has to be essentially OUT.
SSG forms change the quality of the Ki, while SS4 forms turn one's physical limits into it's base.