I don't think you're using units that could allow anyone to detect whether it's caffeine quantity or bioavailability that's at play. It depends on how you dose your espresso and drip coffee. I don't drink espresso but I think people are dosing something like 20 grams per shot, whereas for pourover they're dosing 18 grams of water per gram of coffee. For a standard "coffee cup" (which is 5 ounces of water that results in 4 ounces of coffee = 148 g of brew water), you're using 8g of coffee. Where I'm going with this is if your portafilter takes 40g of coffee for a double shot espresso, that is more coffee than 32g of coffee you'd use to brew 4 "cups", so you're just drinking more coffee. But, standard cups are a dumb unit (my coffee mug holds 400mL of water), so it's possible you aren't using that, and the espresso has a better caffeinating effect for the same amount of caffeine. We can't know because the words and units are conflated in so many ways (there's a cup, which is a drinking vessel, there's the cooking cup which is like 230mL, and there's the coffee cup which is 148mL of water in to get 118mL of coffee out).
I'm not a stickler for metric vs. imperial (I use both), but "cup" is a terrible terrible unit that needs to die. Use grams for anything coffee-related.
> "cup" is a terrible terrible unit that needs to die
Yes. Or more to the point, all of them are; not least because of the existence of all the others.
I know it's rarely going to actually matter that much even when I try to follow a recipe (which to the letter and quantity at least, I generally don't) whether '1 cup' is US legal, Imperial (UK but not common), metric, Canadian, US customary, or any of the other country variations, but it's still infuriating!
I resisted owning any at all for a long time, but eventually gave in thinking it would be a handy way of having volumetric 1:1 ratios if nothing else (e.g. as much tomato as onion) and double as 'a cup' for following American recipes. But which 'cup' did I get? (Bought in the UK.) Nuts. I then realised I already had cup marks on a glass measure (that I'd only used ex ante for millilitres, like a sane person) - sure enough, different 'cup'.
Well, since the recipes you wanted to use were using volumetric ratios then it doesn't matter what size cup you use… until you have something like tsp or tbsp in the recipe, which are more standard (although, because I wrote it I thought I'd check and it turns out that Australian tablespoons are 25% bigger than UK/US/CA's!)
For some recipes those kind of measures are helpful though, so I wouldn't get rid of them. I'll always have a cup handy (a British cup is, handily, the same amount most people would make for a nice cup of tea:) but I won't always have scales.
I'm not a stickler for metric vs. imperial (I use both), but "cup" is a terrible terrible unit that needs to die. Use grams for anything coffee-related.