it would be quite hard. 3/4 essentially builds on 1/2 as much as methods 3/4 builds on 1/2.
Your knowledge of stoich, acid bases, redox reactions all that jazz is heavily featured in 3/4. Really theres only two new things in unit 3 (spectroscopy ajnd biochemistry) and in unit 4 although it is mostly new, it is again, heavily calculation focused. However, if you work hard in unit 3, unit 4 is a lot easier.
Having said that, i completley disregarded chem last year, basically crammed the night before tests and im doing realtively fine so far this year. So a 30 would definately be achievable one would think; you'd just have to work extremely hard at the start of year to ensure that stoich calculations become almost second nature. Because if you get that down pact, thats basically 80% of exam questions nailed.