If you do some math ...
I'm no math whiz, but the last time I checked, one 7" sock is equal to 3 x 4" socks, give or take

My tank is med-heavily stocked, I'd say it's fairly dirty when I change it out pretty brown in colour but not to the point that the sock is overflowing

Yes I think it's +\- 3:1 generally but the space for a 7" is more than a 4" so my question is if 2x,3x,4x of the 4" is enough for a 125g, not simpler....

Yes it would be enough so if you want to go with 4" try to find the poly mesh socks versus the felt as they dont clog up as fast, easier to clean and still do the same job as the felt.

I'm running two 7" socks on a heavily stocked 180. I have a 6' sump so I have plenty of space.

Of course there is something no one has mentioned yet. Those damn 4" socks are a pain to turn inside out to clean or maybe I have fat hands lol. I find the 7" sock on my 60 gallon so much easier to clean!

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WORD !

i cut the plastic inner brace thingy off mmake it much easier . but still a pain.
and the long version of the 4" model is really hard to turn inside out

The 7" mesh socks from BRS are awesome!

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I don't turn my filter socks inside out anymore. I found it made no difference.
Once they're dried, they go in as is.

Stated it twice that the 7" poly mesh are WAY easier to clean, just rinse under a tap and throw it back in.