How you will answer these questions will depend on your own religous views. However, regardless of your own views I think you need to keep things fairly simple for a 4 year old, so you don't scare or confuse your child.
If you aren't sure of your own views on this subject, and are open to hearing some non-standard answers, I would recommend reading The Fun of Dying (in paperback or Kindle). Very interesting book.
My answers would be:
1) God loves us very much. We are here on earth to learn. Sometime we have to say goodbye to our loved ones but we will see them again. It is ok to be sad, but our loved ones are in a wonderful place and want us to be happy.
2) I belive in heaven it seems very much the same as it is here, we seem solid just like here on earth.
3) The person's spirit or soul is energy - it is the real part of the person. Their energy goes to heaven, which for the spirit/soul is like switching to different channel on a TV. Their energy is now on the heaven "channel". Friends and loved ones who are already there help guide a person to heaven right after death.