Timing is important. Spanish Flu was more in 1918, so if you get a 40% death rate, not a 3% death, it will effectively end the war. Too many governments were near collapse. Best guess is a status quo antebellum on the war borders, and each country desperately tires to deal with the aftermath and trying to keep its country from breaking apart.
But you listed 1919, so lets run with it comes back 10 times worse. Many things happen. First, most political leaders and generals are in over 50, and the death rate is much higher, so say 2/3 of the leaders die. The ToV conference collapse due to too many dead leaders. No one is going to have the will to enforce any harsh peace deal. Lower quality land will be abandoned such as in the black death, so I am not sure the border disputes from OTL will exists. With 4 million dead in Paris, how important will A-L be? Either the French or Germans will have plenty of housing to move the entire population out of A-L. Will the Czech really care so much about the Sudetenland if 4 out 10 people in Prague are dead? Can the Red/White fight a civil war with such a epidemic going on? My guess is all the fighting stops due to an inability to fight.
Now an interesting side effect will be the hunger in Europe. Europe will have a much higher death rate than places like Africa or Latin America due to malnutrition. Many will see it as gods punishment for the War. Interesting religious developments.
With so many dead, I have trouble seeing a WW2. It will take a few generations to replaced the lost dead, so their will be plenty of living space for everyone. And likely, central governments will be too weak to hold minority ethnic groups against their will, so Germany likely gets most of the German speaking areas. Posen likely goes to Poland, etc.
The colonial empires will collapse due to weakness of European countries from the death an the plague.
Serbia will be interesting with an over 55% population loss in the war.