I have an expired A-2 visa but will get a G-4 visa in Canada, can I come back to US with an approved 245 (i) status?

245i eligibility means that you can adjust status within the US but if you depart the US, you will lose your 245i eligibility.

You should wait until you have your legal permanent residency approved before you leave.

I don't understand the exact facts of your case, but if your adjustment application has already been approved and you have been granted permanent resident status, then you don't need to have a visa to reenter the US. All you need is a valid passport and your green card.

A visas and G visas reflect someone who is somehow involved in some type of diplomatic functions. And ?245(i) is a provision of the law which generally permits an individual who is not otherwise eligible for consideration of an application for adjustment of status to file the application notwithstanding their ineligibility to do so. Sounds kind of complicated and I would suggest that you consult with an attorney who can figure out the ins and outs of your situation and provide you with a response that will lead you down the pathway to legalizing your status in the US. A and G visas are not all that common because of the diplomatic status that comes with the visas, so finding someone who understands the visas is important and I would highly recommend that you find the right person to answer your inquiries and that you do not rely on someone who is inexperienced or does not understand that situation that you have. Good luck.