1. Does only East and North facing houses are good?

No, All directions are good.

2. Do I need to have threshold at the main door of my house?

Yes, it is belived to stop bad energies entering your home.

3. I am a tenant, will vastu of the house will affect me?

Yes, Vastu will affect you even if your are tenant.

4. Is vastu religion specific?

No, Vastu shastra affects people irrespective of religion, cast, creed or color.