Five slots really doesn’t seem like enough cards for the new Horde mode. There’s a lot of cards I can’t imagine ever having room to equip, and a lot of cards that are dependant on having other cards equipped; for example, a card that gives you a bonus while you have stim, so you pretty much have to equip another card that will actually give you stim. That restricts build options. On top of that, some classes just have an absolute ton of cards, way more than any other class had in Horde 3.0. I think just one more slot unlocked at level 14 or so would make a huge difference.
To be clear, I think Horde 4.0 at this point is a major improvement over Horde 3.0. I love have there’s no literally essential class like the Scout was in 3.0, and it’s perfectly possible to win without an engineer class. It makes the whole thing much more enjoyable with randoms. Still, the card crunch is real; another slot would be a major improvement.