Sorry, I don’t have them handy but I own both of them. The ECC-4 is HUGE. It easily holds a RYTM mkII or A4 plus decksaver lid plus a ton of cables on the interior compartment (or two Digitone/Digitakt/HeatMkII units side by side). There’s a zippered cabled case on the inside of the main compartment, a padded laptop section behind it (accessed with a side zipper), and the generous front pocket which easily holds an Eventide Factor series pedal and related cabling.
The ECC-3, by contrast, is diminutive. It will hold in the interior, tightly, one MkI unit or one Digitone-sized module plus cables. The front pocket might hold a factor series pedal but I think you’d be pushing it if it even fit - it’s more sized for the mid-sized Strymon or Meris pedals. My H9 which is rather fat, barely fits, and there’s not much room for cabling.
However, if you’re carrying just a 'takt or 'tone, the ECC3 is perfect as it will hold your cables, a single effect or smaller outboard piece of gear in the pocket, and your unit plus the decksaver, and be easily transported.
The ECC4 is on the bulky side and I really wish they’d made it a backpack as well as a sling bag - it’s slightly too large to be a comfortable sling.
If you’re carrying a Mk1 product, you have two choices: go with the ECC-3, fit the cables in the front pocket and the product in the main compartment, and that’s IT. Small, compact, perfect, but nothing extra. It’s small enough that it isn’t awkward to carry your normal backpack too, if you’re a backpack sort of person.
Or you can slap a laptop, your unit, all the cables you need, an extra effect or whatever, and still have room leftover for your daily carry in the ECC-4. But that’s going to be the only case you’ll want to take with you.
I also wish they didn’t slap their logos so prominently on the sides of them, as they scream Steal Me, but that’s easily fixed with an iron-on decal…
The materials seem waterproof-ish, as there’s a heavy PVC backing on the outer fabric, but the zippers are clearly not waterproof, so I’d call them “rain resistant” in total, and I wouldn’t deliberately leave them anywhere they’re definitely going to get soaked. However, I wouldn’t panic if I got rained on for a few minutes enroute to a gig or a practice session.