The Commission said its initial investigations found the four platforms had not put in place "appropriate and proportionate measures to ensure a high level of privacy, safety and security for minors".
It said the platforms also do not appear to be abiding by requirements for porn sites to use age verification tools to protect children from accessing adult content.
A Commission official said that "click away" pop-ups currently used by some porn sites, asking users if they are over 18, may not be an effective means of doing so.
The platforms were also found not to have put into place "risk assessment and mitigation measures of any negative effects on the rights of the child, the mental and physical well-being of users," it said.
It comes amid wider scrutiny of online pornography services worldwide, with many regulators looking to crack down on those that do not have age verification in place.
The UK's online safety regulator Ofcom recently announced two investigations into porn sites that did not appear to have any methods to check the age of users.
It said in early May that Itai Tech Ltd - which operates a so-called "nudifying" site - and Score Internet Group LLC had failed to detail how they were preventing children from accessing their platforms.
Pornhub is the most visited porn site in the world - and the 19th most visited on the entire web, according to data from Similarweb.
But it finds itself under increasing regulatory pressure.
It has blocked access to its site in 16 US states, including Alabama, Florida, Louisiana and Texas, that passed laws requiring it to verify the age of users.
It argues age verification should take place on users' devices, rather than on individual, age-restricted sites, to create a simpler process for regulators and enhance privacy for users.