S4 (Hibernate) — RAM powered, CPU off
What it is
S4 is the ACPI power state in which the system has suspended all CPU and GPU activity but maintains power to system RAM. This is the deepest sleep state available in standard x86 and Apple logic boards before full power-down (G3). The system stores its working state in RAM and resumes from that exact point when woken, consuming significantly less power than S3 but more than G3.
In S4, the PPBUS, PP5V_S5, and PP3V3_S5 rails remain powered to sustain RAM and the power management circuit itself. The CPU core domain rail PPVCORE_S0 is off. Wake events (keyboard, mouse, USB, RTC timer, power button) are monitored by the SMC or equivalent power controller, which triggers the return to S0 when signaled.
On Apple boards (MacBook Pro, Mac Mini post-2010), S4 behavior is managed by the T2 Chip or its successor M-series co-processors. On Intel/AMD x86 boards, the chipset SLEEP# pin and SLP_S4# signals drive the power sequencer into hibernation mode.
In practice
Technicians encounter S4 faults primarily as "stuck in sleep" or "won't wake from hibernation" symptoms. A board failing to enter or exit S4 cleanly indicates problems in the power sequencer, SMC, wake circuitry, or RAM power delivery. Common diagnostic approach:
- Stuck in S4: Measure 5V and 3.3V on PP5V_S5 and PP3V3_S5 rails to confirm they remain present. Probe
SLP_S4#or the SMCSLEEPsignal logic state. If rails are dead orSLP_S4#is floating, the power management IC (e.g., ISL6259, TPS51125) is not holding state. - Won't wake from S4: Test wake signals: keyboard matrix continuity,
PS/2_DATAandPS/2_CLKstates, USBDP/DMpull-ups. Scope the RTC oscillator output to confirm the timer is running. Verify SMC is responsive by applying a manual power button press and observingSLP_S4#transition from low to high. - Power drain in S4: Use a DC load or multimeter in current-clamp mode on the PP5V_S5 input rail. S4 standby should draw 50–150 mA maximum on modern boards; above 500 mA indicates a leaking capacitor, parasitic draw, or failed low-power MOSFET in the LDO chain.
S4 diagnostics are often the fastest path to isolating power sequencer or SMC firmware corruption, because the state is simple and predictable: specific rails on, others off.
| Board Type | S4 State | Expected Current | Key Signals |
|---|---|---|---|
| MacBook Pro 13" (2015–2017) | RAM on, T2 running | 60–120 mA @ 5V_S5 | SLP_S4# low, RTC osc active |
| MacBook Air M1/M2 | DRAM + PMU only | 40–80 mA @ 5V_S5 | SLP_S4# low, I2C alive |
| Dell XPS 13 (10th gen Intel) | Platform Controller running | 90–180 mA @ 5V_S5 | SLP_S4# low, EC active |
| iMac 27" (2015) | DRAM on, GPU hibernated | 120–250 mA @ 5V_S5 | SLEEP# asserted, SMBUS alive |
SLP_S4# or ask the SMC to echo its state via serial console to confirm which ACPI state the board is actually in.
See also
Related terms in this glossary:
- S3 (Sleep) — Lighter sleep state with faster wake; CPU powered off but some I/O active.
- S0 (Active) — Full power state; CPU and all subsystems operational.
- G3 (Mechanical Off) — Complete power-down; all rails absent except optional RTC coin cell.
- Power Sequence — The strict timing and voltage-ramp order that gates transitions between power states.