This is so sad!! Maybe they are struggling financially? So they thought they could come to your DS's party but then couldn't afford a gift or a party for their DS.
But yeah the play date thing is strange. I would reach out to them one more time and say your DS is very important to my DS- he constantly asks about him. Please reply and let me know if they can have a play date. And tell them that you also value their friendship.
If they do not reply then I would move on. it is tough because 4yr olds don't understand grown up stuff.