It's been about 4 yrs that I have my green card. But it's about 4 months that I'm living out of US now to stay with my parents in a foreign country.
How long can I continuously stay outside the US to keep my green card valid? What are the rules for that?
There's no hard deadline for you to return to the US. There are some consequences, however, beginning at 180 days, at which point you are to be processed at the border as an "applicant for admission," which will usually have little practical impact on the process.
After a year, your green card is no longer valid for admission to the US. This doesn't mean that you lose your LPR status, and the green card remains valid for other purposes. It just means that you should get a returning resident visa (unless you have a re-entry permit, which you would have to have acquired before leaving the US, so I assume you don't have one).
There are ways to get into the US without a returning resident visa -- in particular, an airline isn't going to refuse to fly you to the US, because they won't know that you've been away for more than a year.
Your LPR status remains until there is a formal finding that you have abandoned your residence in the US. This can be a result of your voluntarily relinquishing it or of an administrative or judicial action. In general, it's probably safe to say, the longer you're away, the more likely the immigration officer is to look into the possibility of abandonment.
If you are absent for more than six months, you may also delay your eligibility to naturalize because this absence would disrupt "continuity of residence."
A good starting point is the USCIS page International Travel as a Permanent Resident, which has more information about these matters as well as links to more detailed descriptions of some of them.
Since this is https://law.stackexchange.com/, I suppose I should add some citations.
The 180-day threshold is found in 8 USC 1101(a)(13)(c).
The one-year threshold is at 8 CFR 211.1(a)(2).
Residence requirements for naturalization are regulated at 8 CFR 316.5. It should be stressed that failing to meet these requirements does not by itself put your LPR status at risk; it only affects your ability to qualify for naturalization.
According to the U.S. Customs and Border Protection's information center:
If you are a lawful permanent resident (green card holder), you may leave the U.S. multiple times and reenter, as long as you do not intend to stay outside the U.S. for 1 year or more.
If you intend to stay outside the U.S. for 1 year or more, you must apply for a re-entry permit with the U.S. Citizenship and Immigration Service (USCIS) prior to leaving the U.S. Re-entry permits are generally valid for 2 years from the date of issuance. Therefore, if you are outside of the U.S. longer than the date the permit was issued, you may be denied entry into the U.S.
To apply for a re-entry permit, you must file an Application for a Travel Document (I-131) with the USCIS. If you applied for permanent resident status, but are not yet officially a lawful permanent resident "green card holder" and you need to leave the U.S. on emergency, you must apply for and receive advance parole to leave the U.S. by filing a I-131 with USCIS.
For additional information, see the USCIS Policy Manual.
If you are required to file documents prior to leaving the U.S., it is imperative that you do so, otherwise, you may be found inadmissible and denied reentry into the U.S.
If you are a green card holder and you do not stay outside the U.S. for 1 year or more, you should have either your green card (I-551) or your returning resident visa to re-enter the United States. You are not required to present your unexpired passport, however it is not a bad idea to carry it with you.