Assuming that you are the primary applicant and the other members of your family are dependents...
In the income/expenditure section of the form should they divide the cost of the trip equally among all four applications?
You can put the entire cost of the trip in your application; there is no need to divide income/expenditure among the various dependents.
For dependents in the family, how does one split monthly expenditure? Technically they dont have an income and rely on the head of the family to provide for the expenditures.
Again, you (as the primary) would reckon the total cost for all your family and use that in your application. There is no point to splitting monthly expenditures by dependent and it would result in fallacious numbers in the first instance.
It's important to link the applications up using the GWF numbers that get assigned as you complete the applications. A precise guide for doing that is at How should a family with young children apply for UK visas? You can follow the steps explained there so that each successive tranche of the family has GWF numbers to provide for the more senior family members.
Has anyone else filled the forms in a similar situation?
Yes, lots of people apply to visit as a family group. It's common, but generalizations can't be made without knowing your nationality and more about your circumstances. I filled them out myself dozens of times (but do not take casework presently).
Final tip: remember to read the guidance carefully before starting.