Selling home to 1 of 4 siblings. How to be fair?

4 Sibs. All are married. 3 Own their own homes and the 4th rents the 2nd floor of our 2 family home. They want to buy it and give us a life estate. We are happy to have this happen. It would give someone to look after us when we reach our dotage. We are in our 70's and in relatively good health.

House is valued at approx. $400k. What would be the value of life estate? That would be subtracted from the appraised value of the house. We would still pay our share of taxes and utility. Should we value the "sale" price based on our paying rent or lower sale price based on our paying no rent?

Original intention was to will the house to the 4 daughters and have them sell the house to divide proceeds. What would be fair to the 3 sibs?

Anything else to be considered?