Zombie Slam is a primary attack.
Primary attack gets 1/2*Str bonus.
PCGen Zombie (all the races I tried, all the versions from 6.05.01 to 6.07.02) are given 2*Str
attacked is a simple Human, starting with Str16 (+2), and the +2 Str from Zombie template = Str 18, +4 bonus.
Slam should be 1d4+6, but PCGen is 1d4+8.
PF Core ruleset (core + bestiary 1)