I don't really have experience with oxy and working out equivalents with suboxone is a little tricky, as it's a funny opioid and sometimes less is more in a very real way. But I have pretty good knowledge of suboxone and I know roughly how much people with a serious oxy habit take vs those with a serious heroin habit take (that's my DOC). I'd say start on 4mg.
Just understand, suboxone builds up in your system, if you take 4mg daily, it means after a couple weeks you will actually have stabilised on a MUCH higher body-load, more like 20mg (from memory, btw). So if it doesn't 'hold' you perfectly on day-1 or day-2, unless you go into violent WD and clearly need more, my advice is to tough it out, you'll probably find you actually feel pretty decent from 4mg, but if you don't - just keep on that dose for a couple days and I'm fairly sure you'll stabilise just fine from that. Don't go on a high dose (8+) unless you really have to, it can be really difficult to come off and in my experience, you can get a lot more adverse affects from those high doses and not necessarily much more benefit (less is more).
EDIT: Oh one other thing, I always kind of just assume people know this, but suboxone will throw you into awful precipitated withdrawals if you take it too early whilst coming off an opioid habit. PWD is really something no one ever wants to go through, personally I've only experienced it with drugs like naloxone after OD'n it it's not fucking pleasant. So, if you're coming off a habit then wait until you're getting WD symptoms before taking the bupe.
Also - I re-read your OP and I realise maybe you're not actually talking about taking sub for maint, in which case, start more like 1-2mg, it's an extremely potent drug. The 4mg recommendation was based on a 60-70mg Oxy/daily habit. If you take too much sub and you don't have a tolerance, you can expect to be vomiting for a couple of days, no kidding.