I guess a good rule of thumb I use is if I am using subs and start to feel w/d symptoms, then I will get something out a full agonist. But if you are only feeling a little sick, then its not much better than just using more sub. Of course the idea is to get the sub out of your system to use the funner stuff, so maybe you accept that its a bit of a waste of your goods, but at least your not feeling a bit crappy waiting to be able to get get a good high. (well either that of dose 2-3 times your usual dose, but thats has risks involved you may not want to take)
Also, 4 mg is not a low dose of suboxone IMO, maybe it is in the suprisingly ignorant minds of the suboxone Drs. Trying cold turkeying off that, I think I'd rather CT off a G of H a day.
I mostly agree with this part of your post. You need to wait until you feel sick from suboxone withdrawal in order to get a proper high from other opioids, and I am not confusing this with having to be sick in order to dose your suboxone after using other opioids in order to avoid precipitated withdrawal.
I had posted my general guidelines on getting high after taking suboxone in other threads. It went something like if you are taking 2mg or below then you can probably get high later on that day (and with anything below 1mg you can possibly get high on top of it), 4mg will usually block for 24 hours or so, 8mg for close to 48 hours, and anything above 8mg will have you waiting close to 3 days before feeling the full high.
If you are taking these doses for maintenance (daily for a prolonged period of time) then the blockade affect will last even longer than stated. This is due to the suboxone building up in your system over time due to its long half-life which will both extend the duration that it blocks for, and increase your tolerance since buprenorphine is a lot more potent than most people realize, which is why I said that I agree with this part of your post in which you say that 4mg is not a low dose, although doctors will have you believing otherwise.
Everybody is different and there are some people that can get high the same day that they took between 2 and 4 mg, but for me I always had to wait close to 4 days before feeling the full effects of another opioid when I was on suboxone maintenance at doses between 8 and 16mg daily. Waiting 48 hours would be like a tease where I could feel a little high for a few hours, waiting 3 days would have my high at around 80% and if I could bear to deal with the withdrawals for any longer than that then I could get the full high.
The way around that, as you stated, is to take moderate doses of the opioid for a few days so that you don't have to experience withdrawals yet not waste the drugs since it's still going to be at least partially blocked. Then when you reach the 3 or 4 day mark you can get a proper high, but if you are serious about your recovery then you won't want to go through all of that effort to get a few decent highs only to have to go through the induction period again in which you have to wait until you are in withdrawals again before starting up with the suboxone again, during which time you have to worry about precipitated withdrawal.
Another thing to keep in mind is that the more you go back and forth between suboxone and other opioids, the harder it becomes to adjust back to the suboxone. Where you used to find that you would feel fine once you started on the suboxone you will now find that the first day on the suboxone won't be fully comfortable, and the more you do this the more days it will take before you stabilize back on your dose of suboxone.
I rarely do the 'suboxone shuffle' yet I now find that it takes 3 days before I feel the suboxone fully working. I won't really feel sick for the first few days, but I won't feel 100% and will experience some chills and minor aches. It's like I don't feel anything from the suboxone, but just won't continue to feel sicker and sicker. Then by day 3 or 4 I will finally 'feel' the suboxone dose, meaning I will feel some positive effects after taking my dose that day.