Father died and left the house in a will to his 4 children along with a mortgage. One of the kids wants to purchase the house. How do the others sell the house? Do they need to go through probate since there was a will. This is in Illinois.

... Do they need to go through probate since there was a will. ...

Yes.

They would file a probate petition whether there was a will. The personal representative (or "executor") would sell the property or, in some states, a sale would have to be approved by the probate court.

A father died left house to his 4 kids. Can we transfer the title to the home to one of the siblings without going through probate?

What State? Probably not.

Please stop opening new threads.

(Please use Answer for more questions on one topic, instead of starting a new post.)

Many states have provision for transferring limited amounts of personal property (A mobile home is personal property, although if the decedent also owned the land, that would be considered real property.) by affidavit. It is a relatively inexpensive and easy way to handle the problem.