During your working days, would you prefer to take a 4000mAh backup battery in your pocket, handbag or backpack?
Or would you rather take a big , powerful but heavy 10000/20000mAh power bank with you?
A small size 10000mAh Magsafe Power bank is at least 200g, While 4000mAh backpack battery is only 95-100g, It is much light.
The 10000mAh Magsafe Power bank is around 115x68x20mm, while 4000mah one is about 78x27*22mm, It is much compact.
So it is more convenient to take the 4000mAh one out; Anyway, 10000mAh is more powerful if you use the C port or USB port to charge.
Or use the ESR HaloLock Kickstand Wireless MagSafe Power Bank which is only marginally larger than the Apple MagSafe battery, but also has a kickstand and is 10,000mAh!