transit to south korea for 4days, do I need a visa?

Yes, you can - although their might be a catch...

As per the websites of the Consulate General of the Republic of Korea in Toronto and the Korea Tourism Organization, foreigners can stay "for up to 30 days in Korea" if they meet one of the following conditions :

• You transit through Korea on your way to the United States (excluding Guam and Saipan), Japan, Canada, Australia, and New Zealand and hold a valid visa (including re-entry permit, but excluding e-visa or Japanese Group visa) issued by the aforementioned countries.
• You transit through Korea on your way to a third country or the country of nationality via direct flight from the United States, Japan, Canada, Australia, and New Zealand to Korea after legally staying in these respective countries.

In your case, the second of these would be relevant to your situation as you are returning to your country of nationality, and you were legally resident in the US via your permanent residency.

Both websites confirm that Permanent Residency is covered under this rule :

Simply put, if you have a visa (study permit, work permit, permanent residency, etc.) from United States, Japan, Canada, Australia or New Zealand, you can transit through & stay in Korea for 30 days, but Korea may NOT be your final destination.

The complicating factor here may be that Timatic, the system used by most airlines to confirm visa requirements, states this rule differently :

Nationals of Philippines with a visa issued by Austria, Belgium, Canada, Cyprus, Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland (Rep.), Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, New Zealand, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, USA or United Kingdom and a confirmed onward ticket for a flight to a third country within 30 days. They must be returning from the country that issued the visa.

As a US Permanent Resident you may or may not have a "visa" for the US. If you do not, then you may strike issues when attempting to check-in for your flight if the airline staff take the Timatic rule literally.

I would suggest taking a printout of the pages linked to above that state that a Permanency Resident does indeed fall under this rule in case you experience any issues at check-in. Once you arrive in South Korea you can expect the immigration staff to be very aware of the rules so there should be no issues there.