Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
Activate only when your opponent Summons a monster. Special Summon 1 Level 4 or lower monster from your hand in face-up Defense Position. Return it to the owner's hand during the End Phase.
--スリップ・サモン
--Slip Summon
local s,id=GetID()
function s.initial_effect(c)
	--Activate
	local e1=Effect.CreateEffect(c)
	e1:SetCategory(CATEGORY_SPECIAL_SUMMON)
	e1:SetType(EFFECT_TYPE_ACTIVATE)
	e1:SetCode(EVENT_SUMMON_SUCCESS)
	e1:SetCondition(s.condition)
	e1:SetTarget(s.target)
	e1:SetOperation(s.activate)
	c:RegisterEffect(e1)
	local e2=e1:Clone()
	e2:SetCode(EVENT_SPSUMMON_SUCCESS)
	e2:SetCondition(s.condition2)
	c:RegisterEffect(e2)
	local e3=e1:Clone()
	e3:SetCode(EVENT_FLIP_SUMMON_SUCCESS)
	c:RegisterEffect(e3)
end
function s.condition(e,tp,eg,ep,ev,re,r,rp)
	return ep~=tp
end
function s.condition2(e,tp,eg,ep,ev,re,r,rp)
	return eg:IsExists(Card.IsSummonPlayer,1,nil,1-tp)
end
function s.spfilter(c,e,tp)
	return c:IsLevelBelow(4) and c:IsCanBeSpecialSummoned(e,0,tp,false,false,POS_FACEUP_DEFENSE)
end
function s.target(e,tp,eg,ep,ev,re,r,rp,chk)
	if chk==0 then return Duel.GetLocationCount(tp,LOCATION_MZONE)>0
		and Duel.IsExistingMatchingCard(s.spfilter,tp,LOCATION_HAND,0,1,nil,e,tp) end
	Duel.SetOperationInfo(0,CATEGORY_SPECIAL_SUMMON,nil,1,tp,LOCATION_HAND)
end
function s.activate(e,tp,eg,ep,ev,re,r,rp)
	if Duel.GetLocationCount(tp,LOCATION_MZONE)<=0 then return end
	Duel.Hint(HINT_SELECTMSG,tp,HINTMSG_SPSUMMON)
	local g=Duel.SelectMatchingCard(tp,s.spfilter,tp,LOCATION_HAND,0,1,1,nil,e,tp)
	local tc=g:GetFirst()
	if tc and Duel.SpecialSummon(g,0,tp,tp,false,false,POS_FACEUP_DEFENSE)~=0 then
		local fid=e:GetHandler():GetFieldID()
		tc:RegisterFlagEffect(id,RESET_EVENT|RESETS_STANDARD,0,1,fid)
		local e1=Effect.CreateEffect(e:GetHandler())
		e1:SetType(EFFECT_TYPE_FIELD+EFFECT_TYPE_CONTINUOUS)
		e1:SetCode(EVENT_PHASE+PHASE_END)
		e1:SetCountLimit(1)
		e1:SetProperty(EFFECT_FLAG_IGNORE_IMMUNE)
		e1:SetLabel(fid)
		e1:SetLabelObject(tc)
		e1:SetCondition(s.retcon)
		e1:SetOperation(s.retop)
		Duel.RegisterEffect(e1,tp)
	end
end
function s.retcon(e,tp,eg,ep,ev,re,r,rp)
	local tc=e:GetLabelObject()
	if tc:GetFlagEffectLabel(id)~=e:GetLabel() then
		e:Reset()
		return false
	else return true end
end
function s.retop(e,tp,eg,ep,ev,re,r,rp)
	Duel.SendtoHand(e:GetLabelObject(),nil,REASON_EFFECT)
end