The problem is to be resolved by the tenants. You are owed £ 370 pounds. If this were 1000 miles away, you would just want the 370. How they divide up -whatever algorithm they choose - is not really your issue, provided that you get your £ 370 (minus whatever you, out of the goodness of your heart being so compassionate, are willing to forego.)
My other feeling is that on a month to month set up, they are equally responsible for 1/4 of the monthly rent. If the remaining two, and the leaving two agree to another payment set up, so be it.
To make this more compassionately complex, suppose it wasn't the mother that died, but one of the renters -what would the payments be then? Now let's suppose it happened a: on the 2nd day of the month, or b: the 29th of the month. Would that change your expectations?