Unit 1 and 2 has only basic concepts that need to be known for units 3 and 4. Understanding of the phonetic alphabet as well as sentence structures and functions are all I can think of right now. These topics (other than phonetic alphabet) should be revised at some stage during unit 3 and 4 anyway. So I don't see any problems with doing units 3 and 4 without 1 and 2.
Ditto. Of Units 1 & 2, Unit 1 is the most important and definitely the most relevant for you to progress through to the final two units. I just did Unit 1 and basically to be set for Unit 3, all you need to be familiar with is::
1. All the terms and definitions on page 13 in
Heinemann English Language VCE Units 1 and 2 (eg. "discourse", "metalanguage", "lexicon", "syntax", etc).
2. The subsystems of language.
3. The prosodic features - stress, pitch and intonation.
4. Word classes
5. Phrases & clauses and the different types of clauses.
6. Sentence types (declarative, imperative, exclamative or interrogative) and sentence structures (simple, compound or complex).
7. The major functions of language (emotional expression, social interaction, recording the facts, etc)
8. Audience, context and mode
9. Primary modes of language - speaking, writing and signing
10. Register and principle of appropriateness
11. Differences between speech and writing (this is in Unit 4 though)