Wow, that's a sack of cats question
. Really, personal preference. Some prefer deeper beds for critters and some like the shallow beach look (vacuum fun). Mine is like yours about 3-4 inches I would guess, I just dumped in 80lbs and said damn that looks good and height is sufficient...........I have the goby/pistol pair and they like the depth. I like the burrowing critters. I do not feel the need to disturb the anaerobic and anoxic bacteria processes in the sand, a light top sand dusting maybe. But I don't mess with it. It can be like creating a mini cycle if I am correct.
There are definitely people way more experienced than I am on these forums on this topic. So, I can really only speak from what I researched when I was doing this as I have a shallow bed, but I have read quite a bit on this and what mtsully wrote was going to be my answer.
It seems to be a heavily debated topic what sand depth is best and what are the real pros and cons. As a result, I can't think of why someone would categorically state a 3 - 4" sandbed is 'bad". It seems like there are pros and cons and people do both with success.
I would assume their concern is that it might build up and eventually release toxins in the deeper part of the sand bed if disturbed. That seems to be the major concern I read about anyway. It's unclear to me if that concern is legitimate or theoretical and I have no first hand knowledge to rely on. I've read well qualified expert opinions that imply that's all bunk and I've read opinions that it's the cause of Old Tank Syndrome and should be guarded against.
I guess the bottom line is that if I intended to have a goby/pistol, I'd have a 3 - 4" sand bed based on what I know now. I don't see any basis I've read to say that having a sand bed of that depth is categorically wrong. People seem to do it regularly on purpose and it works well for them.
See if anyone else has a different view. I can only speak from what I've read, as mentioned. My personal sandbed is shallow.