Rhinoking,
From a teacher's perspective, I'd argue that units 3 and 4 are largely based on units 1 and 2 so, if you already have a firm grasp of 1/2 Chemistry, 3/4 isn't actually that much of a step up -- it just builds on what you already know. Most topics like gravimetric and volumetric analysis (read: stoichiometry), organic chemistry, electrochemistry, are re-visited from unit 1 or 2, so you're already familiar with them. You'll use many of the same skills and knowledge you've already learnt. This is especially true if you've done biology, which will make the discussion of biomolecules and DNA in unit 3 much easier.
On the other hand, from a student's perspective and in terms of assessments, well it depends on how hard the assessments were at your school in units 1/2. By now you've probably looked at the 2013 sample exam so that will give you some idea of what will be expected.
I hope that helps. Good luck with it!