[vitorrossi]

Instead of using

RegisterHam( Ham_Item_AddToPlayer, "weapon_knife", "OnAddToPlayerKnife", .Post = true );

I just tested

public plugin_precache( )
{
    register_message( get_user_msgid( "WeaponList" ), "msgWeaponList" );
}

public msgWeaponList( msgID, msgDest, id )
{
    if( get_msg_arg_int( 8 ) == CSW_C4 )
    {
        set_msg_arg_string( 1, "weapon_tripmine" );
        set_msg_arg_int( 3, ARG_BYTE, get_pcvar_num( g_pTripMax ) );
    }
}

And it works the same way. Is there a reason why you chose to hook Ham_AddToPlayer and send message as opposed to changing the args in WeaponList itself?
Also, WeaponList seems to only be called once when map loads. Message can be unregistered in plugin_init