It was announced in June that Croteam is working on Serious Sam 4. To help fund development croteam released the Serious Sam Humble Bundle which all purchases will go towards game development. An image which teased info about Serious Sam 4 shows the remains of the Eiffel Tower in the background hinting that the new installment will be set in Paris, France. /Note: This image was part of the Serious Sam 3: BFE Deluxe Edition Art Book, it is not specially created for Serious Sam 4 announcement./
The only sure thing for the moment is that Serious Sam 4 will not take place in Egypt.
It is worth noting that the game will not take in Paris. The poster used for Serious Sam 4 is one of many concept posters of Serious Sam 3.
On September 25, 2015, Croteam announced that Jonas and Verena Kyratzes will be writing the story for Serious Sam 4. Later, Jonas Kyratzes wrote on Facebook the following statement in response to a timeline question : "OK, I can officially tell you that Sam 4 is set on Earth, and given what happened to Earth at the end of Sam 3...." seemingly giving the indication that Serious Sam 4 is going to take place before Serious Sam 3, during the war. This statement was later confirmed by Croteam on December 25th, indicating Serious Sam 4 is a prequel to Serious Sam 3.