Chem 3/4 does depend on a good foundation from units 1/2. However, there doesn't seem much point in spending valuable motivation levels covering 1/2 stuff. The stoichiometry, mole, acid-base, redox and gas laws will be very useful knowledge from units 1/2, but it will be covered in 3/4. Thus perhaps head straight into units 3/4. Or, you might just want to do 1/2 and not do anything for 3/4.
On the other hand - keep in mind that overworking during the Summer can sometimes have negative effects. It is possible to do well despite summer study if you are a consistent worker. I did zero work for Chem during the holidays, and really stressed out because I neglected it so much. I had a reasonably good understanding of units 1/2. I think this helped me keep focused and motivated throughout the year. I ended up with rank 1 =)
So, in a way it's also good to enjoy the holidays because you tend to burn out less quickly. In another way, I think I'd have saved myself a lot of stress around midyear exams and term holidays had I done some work for it over the Summer.
Unit 3 is VERY busy, and things will get very stressful. I found it especially so because I didn't finish the course over my holidays, and I'm used to doing so. You could complete unit 3 instead.