I was browsing net found out your concern might be helpful to solve your problem.
Smorkos is correct. You should have consider by calcualting the actual short circuit current on the system(sym & assym value) and the bus bar holder. This is the basis for calculating the mechanical strenght of the bus Bus bar calcualtion to be safe and protect for max. short circuit current (KA) during fault condiotn. The formula is not easy you have to consider the following items:
EMF between bus bar
Bus bar arrangement
Mechanical forace action supporters
mechanical force or tensile strength on bus bar
flexure of bus bar
Those mentioned above shall be consider in calcualting the bus bar
TIp: Vendor has data and formula of bus bar size with respect to the total consumer Loas (amps)
As i can see and experience 35*4 is economically with a safe factor of 152% based on the Transformer rated current.
Hope this will help