hey Yemily!
i'm afraid i can't really help with the 3/4 chem situation
as i'm doing it this year (and struggling lol) but i can give you some (hopefully useful) tips for deciding whether or not to do bio 3/4
i did bio 1/2 last year and i struggled heaps (mainly because of my teacher), and i really didn't understand a lot of the content. however, when i got to 3/4 bio, i found out that pretty much all of the content (so far at least) is new (or just polished versions of 1/2 content)
to give you a bit of an insight into the 3/4 bio topics
unit 3- review of cells (you get an opportunity to relearn or catch up on the basics of bio at the beginning of the year!)
- genes and gene regulation (i'll admit, this topic is kind of tricky, but it is manageable!)
- cellular respiration and photosynthesis
- cell signalling
- immunity (okay this topic is pretty content heavy ngl, but as it can relate to our everyday lives i find it pretty interesting!)
unit 4- dna technology i think (?? sry we are just starting it!)
- evolution (should be stuff related to year 10 science!! <for our school anyway>)
with all that being said, bio 3/4 is quite manageable if you can find the time to do that extra work! also: how are your teachers? as it also helps to have a fab teacher!!
but i think if you're aiming for a high score and you don't want to suffer too much, maybe opt for the uni extension subject if that's what you're after! BUT i don't know much about uni subjects, so i'm not sure about doing 3/4 chem w/ uni chem
i hope that at least a teeeeensy bit of this was useful (or if none of it, sorry!!
) and good luck with your decisions! <3