If I am a level 6 druid but I use a level 4 medium form, do I only get the +2 bonus to STR and +2 to Natural Armor, or could I be a level 4 medium form and get the +4 STR, -2 DEX, and +4 to Natural Armor?
Short answer: no. The beast shape spells specify that you only qualify for the ability score adjustments if you assume a form of that specific size. You do still gain the additional benefits (faster fly speed, more special abilities, etc) if the form you assume has them in its stat block.