I think it depends on a few factors - the course you are doing, whether you're on or off-campus and your family support.
I did Commerce a while back - off-campus, with two children (three towards the end), working part-time (full-time towards the end) with good family support and childcare.
Personally 2 - 3 units per trimester was ample for me. Four units would have tipped me over the edge
My advice would be - enrol in four, gauge your capabilities before the Census date then, withdraw from one Unit until you settle into your studies. I personally don't think it's a good idea to skip any learning material when you first start, you need to suck everything up. After a while, you'll become more savvy with 'pruning' the material. I found reading chunks from a text book somewhat pointless, I found more value in tutorials, lectures and readings. But, that's me - giving me a text book and saying 'read chapter 1' is just parroting information, I didn't seem to learn a lot this way.
You'll nut it out after a few study periods.