More information is needed. Do you mean that they plan to live in their homeland or simply travel 12 weeks each year? In general, too much travel outside of the U.S. to a home country may jeopardize lawful permanent resident status, among other activity. If you have concerns, I strongly recommend an appointment or teleconference with a competent and experienced immigration attorney.
They are eligible for citizenship if they can prove that they have stayed in the U.S. more than half the time (of the last five years). If they leave twice per year for 4 weeks they should be okay.
Normally if they only leave the US twice a year for six weeks at a time, that will not be an issue. This shows they reside a large majority of the year in the US and my guess is they will be able to document their intention.
Generally, the impact of traveling out of the country will not be a problem unless USCIS determines your parents have abandoned their permanent residency.
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