Well if your ALT wasn't in an abnormal range and the rest of your liver tests were fine, then you're ok. What is a dose that is damaging to the liver varies from person to person, but 4 g a day for a lengthy period of time is, for most people, close to the limit. You should probably switch to 10/325 if you see yourself increasing your dosage.
You might want to also add NAC (n-acetyl-l-cysteine) to your meds regimen. That's what they give you in the ER if you go in overdosing on acetaminophen, but it's also sold in capsule form at health food stores and GNC. Milk Thistle Extract (silymarin) seems to work well from what I've read, but I've never used it myself. The NAC will go a long way in preventing liver damage, believe me.
Ideally, a pain management doctor who will prescribe OxyContin is your best bet, as you cannot remain on high doses of APAP for a long time. In other words, you can be doing a lot of damage without any obvious symptoms.
I'm not a doctor yet, just a med student, but this is what I know so far. I understand how much easier it is to use an OP than to go through the hassle of PM doc after PM doc, but it will save your liver and save you money in the long run.
Good luck, and feel free to PM me if you want to talk in more detail.