Why do I read 1/4 gravel max?

The amount of substrate you put in an aquarium depends on what you are doing. I landscape my tanks and create hills and valleys. In my 130 liter tall tank I currently have around 20 kilograms of 1-2 mm gravel (almost sand) and a further 30 kilograms of rock, added with a 10 kilogram piece of driftwood and around 40 live plants. This leaves around 80-90 liters water capacity. Perhaps the 1/4 gravel max warning is for those that are unaware that items in the tank lowers water capacity. 1/4 of total tank depth is very deep substrate and could possibly lead to anaerobic bacteria problems.

The only possible thing that I can guess that the 1/4 max refers to only fill the tank one quarter of the way full with gravel or less. It doesn't matter though because you normally put between 1 and 2 pounds of gravel per gallon of tank as you read (depends on the tank shape too). There would be some exceptions where more gravel (or other substrate) might be used; live planted tank or for some types on saltwater fish or invertebrates for example. As long as the other aspects of the tank are correct, an inch or two of gravel should be fine for a crayfish until it is big. If the tank isn't cycled yet, or if you don't know what I mean by cycled, do some more reading on aquariums before you get any animals for it.

Going by gallons isn't really a good strategy- a tall tank with the same footprint as a short tank would have way more substrate for no apparent reason in that scenario. I usually use about an inch or two which is enough to cover the bottom and hold decor in place but not so much that it becomes a trap for waste and hard to clean.