The boldfaced part is incorrect.
UE4 developer here. Unreal Engine licensing explicitly said UE4 is not open-source, and it is not compatible with GPL, LGPL, and similar derivative licenses. It is also not free, because you have to pay Epic a royalty equal to 5% of all worldwide gross revenue actually attributable to each of your product, regardless of whether that revenue is received by you or any other person or legal entity. If it's free, you wouldn't even be paying for the royalty fees in the entire lifetime of your sales product. It is also not free because you may be subject to a fee in which you are paying for paid contents.
For more information on the royalty fees, please check the EULA section #5,
Royalty, in regards to what fees you may be subjected to.
Source - Unreal Engine EULA