The first thing you do when you determine how big your dice pool is, is to apply terrain modifiers. Hence, if you should roll 4 dice and you're attacking a unit in a forest hex, this amount drops to 2. After that, you apply all the other modifiers, in the order of your choosing. So, a single-figure blood harvesters unit attacking a unit in a forest hex would roll 4 dice, 2 for the normal attack, +1 die/figure missing.
Beware that flying creatures follow different rules, as specified on pag 4 of the reference guide