As with all derailleur systems, the gearing on your 4 speed Brompton will require adjustment from time to time, particularly within the early life of the product when the cable can stretch, to ensure it performs optimally.
The most common cause of gear issues is that the cable tension is not correctly set; usually caused by 'cable stretch'. You can increase the cable tension by winding the cable tension barrel adjuster outwards. Shift through the gears until you are happy that the gearing is sitting in each gear correctly and running through the gears in a smooth fashion.
Brompton Interactive 4 speed Gearing Guide
Click through the easy to use guide to diagnose and fix your Brompton Advance 4 speed gear system