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I bought recently a e-bike (urban/light offroad, front suspension). The spec sheet indicates that the brakes are Magura MT5 eSTOP. I noticed that the front calipers have 4 pistons, as one should expect from the spec sheet, but the rear calipers have 2 pistons. Judging by the colours of the "piston ornaments" it should be an MT4 eSTOP.

The pictures on the manufacturer website show this configuration, which suggests it's the intention, and not a replacement on my individual bike because of parts availability. The same discrepancy between the spec sheet and the pictures can be seen on other e-bikes from the same manufacturer (a rigid urban bike, and a lightweight hardtail MTB).

Besides the minor cost saving (20€ RRP, but that's for them, not the client) and 15g weight decrease, are there advantages or drawback of using this combination?

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From my experience, the combination is actually really good. I ride (rode) MT Sport (marketing name for MT5+MT4) brakes on two different MTBs and there's a reason for that combination. I think it can be generalized from MTBs to road bikes.

While the MT5 ist quite strong, it can be a bit digital (hard to modulate). This is a problem especially in the rear, as the braking traction breaks easier than in the front. So the MT4 gives a bit more modulation for the rear, sacrificing some braking power. But as pointed out in other answers and in the comments, the main braking power should come from the front wheel. (70:30, 80:20, whatever)

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Normally, bike brakes are getting about 90% of their performance from the front wheel.

In an optimal perfect world, the front brake would do 100% and the rear wheel would be just on the point of loosing traction with the road surface.

However most countries mandate that a bicycle will have at least two independent brakes, regardless of if its a bike, car, truck. So a lesser-rear brake provides that.

The cheaper brake saved the manufacturer 20 euros? It has likely saved the purchaser double that in the final price. Most bike groupsets are specified to meet a price point, so by increasing one part some other part will be down -graded. Perhaps the saddle, or maybe the chain goes from being XT to Unbranded, or the pedals go from adequate to garbage or get left off completely.

The only times a rear brake is more important than the front are:

  • the front brake has failed
  • the front wheel has lost traction/flatted
  • you're riding on ice and trying to brake
  • you're towing a heavy bike trailer and front braking results in a jackknife.

Road motorbikes don't even have a hand lever for the rear brake - their rear brake control is on the foot. Shows how little use a rear brake is on a heavy/fast motorised bicycle, so an ebike would be leaning in the same direction. (similar to the "ebike tyre markings" question last week.)

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    On the other hand, it is not uncommon for the rear pads and rotors to be more worn than the front ones.
    – Vladimir F Героям слава
    Mar 26 at 20:01
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    Coming from the off-road side off cycling, conditions are per definition less than perfect, and keeping a reasonable balance between front and rear is quite important. Just to be clear, the point of the question was not to whine about a "lesser" rear brake than specified (the case you mention are more about cheaping out on "generic specs"), the two brakes are at the same level, and differ by the number of pistons. There might be nuances in behaviour between 2 and 4 pistons that makes this combination "better" than 4 pistons rear and front, and that is the point of the question.
    – Renaud
    Mar 26 at 20:49
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    I really can't agree with the 90% figure, except in one situation: emergency braking on tarmac under ideal conditions. But even on tarmac one should in general involve the rear brake a bit more, if for no other reason but to keep the front one cooler. And on MTB, tactical use of the rear brake can help a huge amount: with stabilty, because the rear liftoff ⇒ reduction-of-traction effect creates a negative feedback against stoppie/OTBing, and with rider confidence, because a rear wheel starting to slide is a warning/indication for how the front can['t] be loaded under the conditions. ...
    – leftaroundabout
    Mar 26 at 21:35
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    ...70%/30% would fit better, which is of course still enough inbalance to justify putting twice as many pistons on the front. On modern MTBs, dropper posts and slack geometry help actually keeping so much weight on the rear that this also corresponds roughly to the optimal distribution for deceleration.
    – leftaroundabout
    Mar 26 at 21:36
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    The grip on (wet) gravel means that the same arguments apply as in true mountain biking. But whenever grip is limited and you're not assured of braking in a perfectly straight line, the balance shifts towards the back. You still can't brake as hard overall but a rear wheel slide is far more recoverable than a front wheel slide. Of course braking while going straight is preferred and then you can often have enough front grip for the back wheel to lift, even on dirt
    – Chris H
    Mar 27 at 6:59

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