I was wondering the same thing. Both my wife and I are on around 1mg weekly with the aim to turn some fat into muscles rather than purely lose weight. I am still playing to find the sweet spot, likely we will stay around 0.8 - 1.2 for now. With these dosages we did see, particularly in my wife, an increase in heart rate, which according to my research on the internet is an indicator of glocagon response.
The 4mg (and I would prefer not to go that far myself) come from the fact that during the Eli Lilly trials BHB - the ketone the body makes through fat oxidation - became clearly measurable in the participants blood samples, well later than measurable acivation of GLP1 and GIP (EC50 for glucagon activation is nearly 10x that of the other receptors, meaning it is a lot slower to achieve its full (or half in that case) effect. )
One more thing: There is a likely a big difference from person to person. From a ketogenic (ultra low carb) diet that keeps you in a fatburnign state I know that some people are very quick to go into a keto state (= produce BHB) and others, particularly those which are heavily overweight and were used to eating throughout the day) take a lot longer since the body "forgot" how to make ketones. Going into Keto, sometimes adding a BHB salt can help in that situation since having BHB in the blood can support the body to acticate the creation of BHB by the liver. If that is in ANY way at all appliccable for Reta I have no idea, but what I take from that is that bodies react very differntly to a number of factors.