type Test = { id: '1' };
type Result = AppendToObject<Test, 'value', 4>; // expected to be { id: '1', value: 4 }

ことを上手く扱わないといけない

Mapped Typesを使って1つのRecordとして定義する

type AppendToObject<
	T extends Record<string, unknown>, 
	K extends string,
	V
> = {
  [P in K | keyof T]: P extends keyof T ? T[P] : V;
};

条件分岐のある最も無難な定義という感じ

type AppendToObject<
  T extends Record<string, unknown>,
  U extends string,
  V,
> = MergeIntersections< T & { [K in U]: V; } >;

type AppendToObject<
  T extends Record<string, unknown>,
  K extends string,
  V,
  R = T & { [P in K]: V },
> = {
  [P in keyof R]: R[P];
};