Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
Level 4 or higher monsters cannot attack.
--グラヴィティ・バインド－超重力の網－
--Gravity Bind
local s,id=GetID()
function s.initial_effect(c)
	--activate
	local e1=Effect.CreateEffect(c)
	e1:SetType(EFFECT_TYPE_ACTIVATE)
	e1:SetCode(EVENT_FREE_CHAIN)
	c:RegisterEffect(e1)
	--cannot attack
	local e2=Effect.CreateEffect(c)
	e2:SetType(EFFECT_TYPE_FIELD)
	e2:SetCode(EFFECT_CANNOT_ATTACK)
	e2:SetRange(LOCATION_SZONE)
	e2:SetTargetRange(LOCATION_MZONE,LOCATION_MZONE)
	e2:SetTarget(s.atktarget)
	c:RegisterEffect(e2)
end
function s.atktarget(e,c)
	return c:GetLevel()>=4
end