Hi SamiJaneOwen!
As someone who did 7 3/4 subjects in a similar way to what you might be planning (I did 3 3/4's in Year 11 and 4 in Year 12), I'll throw in my two cents:
If I may ask, when did you do your 1/2's for the 3/4's that you're doing? I'm guessing that your VCE program is effectively spread over three years, so I'll put up what I did:
Year 10: 6 1/2's
Year 11: 2 1/2's and 3 3/4's
Year 12: 4 3/4's.
Personally, while I agree that it's very important to be able to focus on your 3/4 subjects and having fewer of them certainly helps with this, I think that the added 'safety net' of having one extra subject outweighs this concern. In my humble opinion, if you're taking 3 3/4's in Year 11, (and maybe doing 1 or 2 1/2's as well), you're definitely more than capable of taking 4 3/4's in Year 12. I found that in Year 12, I definitely had a decent amount of time to try to learn the content of my subjects.
I think that taking any 3/4 unit in Year 11 is a great way to 'test the waters' of VCE so to speak, because after trying the exams and SACs, you'll have a great knowledge of how high the bar is set, what works and doesn't work for you in terms of studying, and general knowledge of how VCAA likes to ask questions on exams. So if you have a score from Year 11 that you might want to improve upon, I think taking 4 3/4's is a good way to ensure that it becomes an increment, or doesn't even count at all!
Also, if you do 4 3/4's in Year 12, and find it's not really working out for you, there's really no reason why you have to devote equal time to all 4 of your subjects! You could effectively focus on only 3 of your subjects, which would have happened anyway had you only taken 3! So to me, it's certainly no worse than taking 3 subjects, but has the potential to be better!
Finally, one last argument I'd like to make is that in this case, taking one more 3/4 means you'll learn more. I know that the ATAR is important, but I personally believe that your high school experience will be made all that much better if you're learning something you really want to, and in this case, have the time to put good effort in. I mean, if there's nothing you like apart from the 6 subjects you're currently doing, then maybe it's worth doing 3 in Year 12. But if there's a subject that you enjoy that you might not be doing because you're worried about time management in Year 12, then I say go for it! I definitely think it will be a plus.
Anyways, I hope sharing my perspective helps, and I wish you all the best!