Quick Answer: A 4-pin Ethernet cable carries two twisted pairs on pins 1, 2, 3 and 6, which is all 10BASE-T and 100BASE-TX need. That caps it at 100 Mbit/s and 100 m, and it rules out 1000BASE-T, which requires all four pairs. Power over Ethernet still works, but only through the data pairs, so 802.3af and 802.3at are feasible while 802.3bt, which needs all four pairs for 60 W and 90 W, is not. If the fourth pair is missing and the pairs are not twisted, expect link failures at distance.
The term covers two very different products. One is a proper two-pair Cat5e or Cat6 construction terminated on the 1-2 and 3-6 positions of an 8P8C plug. The other is flat phone-style cable with four parallel conductors and no twisting at all. Both will carry a 100 Mbit/s link across a desk. Only one of them survives 40 m next to a mains cable.
What 4-Pin Actually Refers To
An RJ45 jack has eight contact positions, numbered 1 to 8 with the clip away from you. T568A and T568B both put a pair on 1-2, one on 3-6, one on 4-5 and one on 7-8. A 4-pin cable simply omits the blue and brown pairs. The connector may be an 8P8C shell with two contacts unpopulated, or a 6P4C plug that will not latch. Check which you have before ordering a mating part.
Why the Ceiling Is 100 Mbps
The pairs 100BASE-TX uses
100BASE-TX transmits 125 Mbaud with 4B5B line coding, one pair per direction: one for transmit, one for receive. Two pairs are sufficient, which is why 4-pin cabling became common for wall runs in the 10 Mbit/s era and stayed common on cheap IP cameras and door stations. 10BASE-T uses the same two pairs at a much lower rate.
What happens when you force gigabit
1000BASE-T runs 250 Mbaud on all four pairs at once, in both directions on each pair, using echo cancellation. With two pairs missing the PHY cannot train, so the link falls back to 100 Mbit/s through autonegotiation or never comes up. Forcing a port to 1000 Mbit/s on a two-pair run gives a link with no carrier.
PoE Over Two Pairs
802.3af and 802.3at define two delivery methods. Alternative A puts 48 V DC common-mode across the data pairs through the centre taps of the magnetics, so the same two pairs carry signal and power. Alternative B uses the spare pairs 4-5 and 7-8, which a 4-pin cable lacks. A two-pair cable therefore supports Alternative A only.
| Standard | PSE output | Power at device | Pairs needed |
|---|
| 802.3af Type 1 | 15.4 W | 12.95 W | 2 |
| 802.3at Type 2 | 30 W | 25.5 W | 2 |
| 802.3bt Type 3 | 60 W | 51 W | 4 |
| 802.3bt Type 4 | 90 W | 71.3 W | 4 |
| Passive 24 V injector | No negotiation | Device dependent | 2 |
Where 4-Pin Cable Is Legitimate
Short camera drops on an existing 100 Mbit/s installation, intercom stations, sensors that only need 10 Mbit/s, and any retrofit where the cable is in the wall and the endpoint cannot exceed 100 Mbit/s. It is the wrong choice for a new desk run, an access point or anything that might later need gigabit, because pulling cable costs more than the copper. A 4-pair Cat6 cable line run removes the ceiling permanently.
The Twisting Problem
Conductor count is only half the story. Balanced signalling depends on the two wires of a pair being twisted at a controlled rate, so induced noise appears as common mode and cancels. Flat four-core cable has no twists and no defined impedance, so near-end crosstalk rises and the link degrades with length and proximity to power conductors. An untwisted pair Ethernet cable may negotiate 100 Mbit/s at 3 m and drop the link at 25 m. Keep flat cable short and away from mains.
Practical Scenarios
- IP camera on a 30 m legacy drop: a 4-pin run carries 100 Mbit/s and 802.3af on the data pairs. Confirm the camera accepts Alternative A power.
- New office desk run: pull four-pair Cat6 and terminate all eight conductors. A 4-pin Ethernet plug on a four-pair cable wastes the pairs and confuses the next technician.
- Door intercom needing 48 V: check the datasheet for the pair assignment. A device wired for Alternative B will not power up on two-pair cable, whatever the injector claims.
- Long run beside a motor cable: skip flat four-core. Use shielded twisted pairs, bond the shield at the switch end, and test with a certifier.
FAQ
Can a 4-pin Ethernet cable carry gigabit?
No. 1000BASE-T and 10GBASE-T both require four pairs, and the PHY will not train with two missing. Autonegotiation will settle on 100 Mbit/s if the endpoints support it. Some vendors sell two-pair cable rated for 2.5GBASE-T, but that standard still assumes four pairs, so treat the claim with suspicion.
Does PoE work on only two pairs?
Yes, up to 802.3at Type 2 at 30 W from the source, because that standard delivers power over the data pairs. 802.3bt Types 3 and 4 spread current across all four pairs; with two pairs the cable would carry the full current on half the copper, and compliant hardware will refuse to negotiate that.
How do I test whether a cable has four pairs?
Look at the plug: eight contacts populated means four pairs. A continuity tester that reports pairs 4-5 and 7-8 as open confirms two-pair construction. The quickest functional check is to force a 1000 Mbit/s link between two gigabit ports and see whether it comes up.
