[
{
"mob": "minecraft:zombie",
"weight": 100,
"mincount": 1,
"maxcount": 4,
"result": "allow"
},
{
"mob": "*",
"result": "deny"
}
]

"mob": "*" is not thing. Remove that line
Also only use maxcount. Remove the mincount line.
Also remove the weight line. That doesn't work in spawn.json