Google, the money-making engine of parent company Alphabet, confirmed a copy of the note published by Bloomberg News but declined to comment further to AFP.
One of the workers fired was connected to a petition condemning Google for working with the US customs and border patrol agency, which has been involved in President Donald Trump's crackdown on illegal immigration.
Authors of the post contended that Google policies on data security were tightened simply to provide cover for getting rid of the workers.
The Google workplace has been disrupted by employee opposition to top-level decisions ranging from forging contracts with the US military to tailoring a version of the search engine for China.
A year ago, Google employees poured out of premises at its Mountain View campus and around the world to protest the company's handling of sexual misconduct allegations.
When they were still suspended prior to the firings, two of the quartet on Friday addressed a protest rally at the company's San Francisco offices, according to US media.
Jeffrey Hirsch, a University of North Carolina law professor and former National Labor Relations Board attorney, said Google could face legal problems unless it could show a consistent approach towards other staff accused of the same conduct as the four.
Google's virtual monopoly on internet searches has provoked accusations that it abuses its global dominance to attract more advertising money at the expense of others, such as the news media.
Last week, the search leader updated how it handles political ads, stepping up actions it says it is taking in the buildup to the US presidential election next year.
Original article