I'm pretty sure they are the same here are the specs:
Reference
ACCESSORIES: 4 different-sized ear tips, storage case, clothing clip
BUILT FROM: 2011
DESIGN: Dynamic Moving Coil Micro Speaker
DIMENSIONS: 8.5mm diameter
DRIVE COMPONENTS: Full range KG 25
FINISH: Black with copper accents
FREQUENCY RESPONSE: 10Hz - 19KHz
INPUT CONNECTIONS: 3.5mm
NOISE ISOLATION: -16
NOMINAL IMPEDANCE: 18-ohm
SENSITIVITY: 110db SPL/mW
WEIGHT: 11.9 grams
S4
ACCESSORIES: 3 different-sized ear tips, storage case
BUILT FROM: 2009
DESIGN: Dynamic Moving Coil Micro Speaker
DIMENSIONS: 8.5mm diameter
DRIVE COMPONENTS: Full range KG 25
FREQUENCY RESPONSE: 10Hz 19KHz
INPUT CONNECTIONS: 3.5mm
NOISE ISOLATION: -16
NOMINAL IMPEDANCE: 18-ohm
SENSITIVITY: 110db SPL/mW
WEIGHT: 11.9 grams
Only difference is colour and extra pair of earbuds