15 µF, 4 V, 4 Ω — what these ratings mean on your BOM
The 15 µF capacitance at 4 V rated voltage with ±10% tolerance puts it in the bulk-decoupling class for low-voltage rails — think 3.3 V or 1.8 V supply bypassing where a ceramic cap's DC-bias derating would eat half the effective capacitance. The 4 Ω ESR (equivalent series resistance) is the spec that matters for ripple current and transient response. At 15 µF, a 4 Ω ESR means the self-heating under ripple is higher than a low-ESR tantalum — budget the ripple current per the datasheet's power-dissipation limit, not just the capacitance. For a 4 V rail pulling 100 mA peaks, this cap handles the transient without issue; above 500 mA ripple, consider a lower-ESR part.
TAJS156K004RNJ carries an active product status from KYOCERA AVX. It is a current-production part in the TAJ series, available through independent distribution channels.
The molded construction and 1206 case keep it usable in outdoor telecom, engine-bay electronics, and downhole tools where the ambient exceeds 85°C. Derate the voltage above 85°C per the TAJ series application note — at 125°C the rated voltage drops to 2/3 of the 4 V mark.