F4C is a comprehensive system for applying patches to the Final Fantasy IV ROM; the main initial output of running the FF4FE generator is a F4C script which in turn is applied to the base ROM to produce the final output. The .f4c file format represents the set of patching instructions to be applied to the ROM.

A .f4c script is parsed using the Lark parser; the technical specifications can be found in f4c/grammar*.lark.

A .f4c script consists of one or more blocks, specified as a block type, any parameters to the block type in parentheses immediately following the block type identifier, and a body enclosed in braces. Comments are also permissible via a double slash //; these can also be used to create conditionals in the FF4FE context specifically.

As an example, here are the first three blocks of FreeEnt/scripts/randomizer_keyitems.f4c:

//-------------------------------------------------------------------
// set up Hook item
consts(item) {
    $FC     fe_Hook
}
text(item name #fe_Hook) {Hook}


//-------------------------------------------------------------------
// make PinkPuffs just directly drop Adamant Armor
droptable($28) {
    common #Ether2
    uncommon #Elixir
    rare #NinjaStar
    mythic
        // %pink_tail_item const%
        #AdamantArmor
        // %end%
}

The permissible block types in f4c scripts are as follows:

The consts block defines constants that can be used by name. The sole parameter is the “family” of constants, which is a namespace used when referring to the constants.

As seen in the above code snippet, references to constants are prefixed with a # character. The #fe_Hook example above could also be referenced via #item.fe_Hook.

The event block defines the script for an FF4 event. The sole parameter is the event ID that the block (re-)defines.

The text block represents a new piece of text to be used. The parameters define which text is to be set - this can be one of the following:

The trigger block overwrites or deletes a map trigger. The two parameters are the map number and the number of the trigger to be overwritten or deleted.

The ai_script block overwrites a monster AI script, The parameters are either a script number by itself for overworld encounters or moon followed by a script number for underworld/moon encounters.

The placement block overwrites or removes an NPC placement. The two parameters are a map number and the placement index.

The map block sets properties of a map. The single parameter is the map number.

The mapgrid block sets tiles on a map. The three parameters are the map number, X coordinate, and Y coordinate.

The npc block sets properties of an NPC. The single parameter is the NPC number.

The eventcall block sets the logic for an event call hook; these hooks are used both by NPCs and map triggers. The single parameter is the event call ID.

The shop block sets the contents of a shop. The single parameter is the shop ID.

The actor block sets the properties of an actor - basically a party member slot. The single parameter is the actor ID.

The droptable block sets the drop table for a monster. The single parameter is the monster ID.

The formation block sets the properties of a monster formation. The single parameter is the formation ID.

The monster block sets the properties of an individual monster. The single parameter is the monster ID.

The spellset block sets a character spellset. The single parameter is the spellset ID.

The patch block directly specifies new bytes to be written to the ROM. The parameter is a ROM address; as with other ROM address specifications this can be for an address in an unheadered ROM (the implicit default if not otherwise specified), an address in a headered ROM, or a bus address.

The msfpatch block specifies new assembly instructions in the msf patch format. This block takes no parameters.

The chr block adds or overwrites text characters. The parameters are a ROM address (usually a bus address) and (optionally) the bit depth of the character data (2bit, 3bit, or 4bit (the default)).

The pal block sets palette data. The parameter is a ROM address.