I checked the contents of both of my 5-packs and they are identical; all have the single thick rubber plug.
I'm not a big fan of syringe-filling but even with the blue adapter tip and tiny holes, it wasn't as difficult as I thought it would be. I chose my go-to juice, an appropriately aged bottle of TopVapor Fried Apple, because I was sure I could taste any subtle difference the E2 may have caused.
At first, the vapor did seem to taste different, not bad but different, from what I had been vaping all day on my Little Chuck. After several puffs I realized the difference was probably the warmer vape and not some off-putting residue or chemical in the carto. Or perhaps I didn't give it time to thoroughly wick.
Overall, the flavor wasn't as sweet as usual, the TH seemed a little more mild yet thicker (if that makes any sense) than I get from a standard Boge 510 carto on the same hardware. It also seemed that the flavor difference related to the heat of the carto. In other words, it was only noticeable after several heavy draws. After giving it a rest, the initial draws were a little sweeter than before, but still warmer and thicker.
I got several hours of pretty heavy vaping off the first fill with no discernible loss of performance or taste.
I also tried it on an eGo. The flavor seemed more typical and the vapor was again warmer and thicker than I get from a typical Boge 510.
I refilled it (took about .5ml), and then filled two more. I plan to let all three set overnight and give them another shot tomorrow, comparing the one I used this evening to the two I haven't touched.
So far, I have nothing bad to say about the E2-R4s and my first experience was basically positive but I'll need to continue to experiment and compare before I take a formal pro, con, or indifferent position.
MadVapes got fresh stock of my old stand-by Boge 510s, and ever the contingency planner, I ordered a few boxes just in case. That's my third MadVapes order in five days. It's a little crazy but I'm racking up some hella points!