4 That's the house that/which/whose I was telling you about.

4. That's the house that/which I was telling you about.

В данном предложении можно использовать местоимение that или which, так как оно относится к неодушевленному предмету (the house). Местоимение можно опустить: That's the house I was telling you about.

Ответ: That's the house that/which I was telling you about.