There was a similar thread posted about a week ago, about the weak output on some Elektron machines. I don’t have an A4, but my Monomachine has the same “issue”.
In the other thread I never saw a definitive answer as to why the Elektron’s output is so quiet, but it is a common issue, and not a broken machine. It must be the preamps inside the boxes.
Some sounds you make will be inherently quieter than other sounds, obviously. Part of the solution to getting your volume level up is designing and EQ’ing the sounds better, so they sit in the mix with more breathing room. Muddy frequencies can cancel out frequencies of the other sounds.
My remedy (as well as other’s have done) was to buy a small but good quality mixer. Most mixers will have plenty of GAIN for boosting those low level signals. Not too mention that the sound IS BETTER when the Elektron machines are run into individual inputs of mixer, versus daisy-chaining them together via the ins and outs of the Elektron boxes.
One last tip: On the Monomachine there is the ability to turn Filter Tracking off. This one thing alone can double the volume of certain sounds, and make them much richer and fuller (still don’t understand why the default is for filter tracking to be ‘on’). I don’t know if the A4 has the same feature, but if it does, try it!