SymPy : Factorisation of a simple expression “a1*a2+a1*a3+a1*a4+a4*a2”

Question

I want to factorize/simply this expression f = "a1a2+a1a3+a1a4+a4a2" to a1*(a2+a3)+a4*(a1+a2) for example (minimize number of operators).