“The new version 4 Raspberry Pi uses more power and uses a USB-C connector to handle it. This means your old RPi 2/3 power supplies aren't suitable.
This is new power supply meets the spec and is very compact. It runs quite cool, even with a USB3 external HDD added to the Pi.”
“After using several variations of the Raspberry Pi, and many many more USB chargers, I have to recommend this product, and in fact all of the official Raspberry Pi charger range. This PSU works brilliantly with my Pi4, which currently has a HDMI to VGA adaptor and a 120GB SSD attached.”
