Apple MacBook Air battery
I also just upgraded my old MacBook Air 2013 (cpu i7-4650U) with MacBook Air 2019 13' (cpu i5-8210Y) and mine new mac shows me 4381 mAh too. I think Apple reduced the battery capacity to decrease physical battery size. Why? If we'll compare this 2 CPUs:  i7-4650U and i5-8210Y we'll see that max TDP (aprox. shows cpu power consumption) is 7W while for i7-4650U it's 15W which is more than twice more. This can explain why Apple reduced the battery capacity almost twice.




CPUs comparison: https://www.cpubenchmark.net/compare/Intel-i5-8210Y-vs-Intel-i7-4650U/3402vs1955