You cant use a literal like that, you have to create a constructor and initiate it like
var VM = function() {
this.isValid1 = ko.observable(false);
this.isValid2 = ko.observable(false);
this.isValid3 = ko.observable(false);
this.isValid4 = ko.observable(false);
this.isAllValid = ko.computed(function() {
return this.isValid1() && this.isValid2() && this.isValid3() && this.isValid4();
}, this);
};
Or set the isAllValid function after the creation of the literal, but i do not think literals should be used for VMs