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Leaving the country for more than 12 consecutive months with your green card can make your readmission to the US difficult. 4 months is not a risk, since you are allowed to leave the country with your green card.
As far as joining the new company, that would depend on any contractual obligations you have with your current company. If you guaranteed them a certain amount of service in exchange for your green card, you have to complete that contract with them or they can pursue you for breach of contract. However, if you have no such contractual restrictions, you can indeed switch over to some new employer within your restrictions.