Hermit crab tank help / review

Hey guys,

I’ve posted on here before but I really need some advice on my tank. I put what I believe to be 6 inches of sub (a mix of coco fiber and play sand) but I’m concerned that it is not the depth of my largest crab. The pictured shell below is the size of my biggest crab (there is no crab in there, just an empty shell for reference). Would you all recommend I add more sub or should it be okay?

I have 4 hermit crabs currently in a 40 gallon (Flynn, Luther, Stanley, and Stevie). I put so much work and time into this tank and I really want to be able to keep all my crabs in here. However, my crabs safety and comfortability is the most important this to me. So, Should I invest in a larger tank or can I keep 4 in this tank? Also, any constructive criticism as to the tank itself would be wonderful. I just made the ladders into the water dishes so hopefully that holds up. Will hermit crabs climb mesh zip tied to egg crate? Also, is it okay if my tank is a bit cluttered? I don’t want my crabbies to feel to stuffed.

40 gallons 4 crabs Coconut fiber and play sand Critter keeper water dishes (salt and fresh/ not sure of the gallon sizes) Moss pit, Lots of foraging materials to the left side of the tank Trying to keep the right side more open. I have a large heating pad/mat that covers the back length of the tank and another smaller one on the left side to keep humidity up and reduce condensation build up (happens most frequently at night when the room temperature drops significantly).

Thanks for everything and happy new year.