Are you sure the LEDs are 5V ? Try first to power the 4 LEDs in series to 12V capable of 2 Amperes, and come back.
12V(+)-------|>|------|>|------|>|------|>|---------(-)
If you want to run all 4 LEDs from a 5V supply, the 5V supply would have to be capable of 11 Amperes, not 2.1A