asrock Fatal1ty Z270 Gaming K6 vs k4

Apart from the general board layout, these are the major differences between the ASRock Fatal1ty Gaming K6 and ASRock Fatal1ty Gaming K4 motherboards:

BIOS:
K6: Supports Dual BIOS (1 Main BIOS and 1 Backup BIOS)
K4: Single BIOS Only

LAN:
K6: Dual LAN with Teaming (1 Intel I219V and 1 Intel I211AT) *Teaming not supported in Windows 10
K4: Single LAN Only (Intel I219V)

PCIE SLOTS and MULTI-GPU SUPPORT:
K6: 3x PCIe3.0 x16 Slots (PCIE2, PCIE4, and PCIE6) running in x16, x8/x8, or x8/x8/x4 modes
K4: 2x PCIe3.0 x16 Slots (PCIE2, and PCIE4*) running in x16, x16/x4* modes

K6: Supports AMD CrossFireX (2-Way, 3-Way and Quad) and NVIDIA SLI (2-Way and Quad)
K4: Supports AMD CrossFireX (2-Way and Quad) only *NVIDIA SLI not supported

K6: 3x PCIe3.0 x1 Slots (PCIE1, PCIE3, and PCIE5)
K4: 4x PCIe3.0 x1 Slots (PCIE1, PCIE3, PCIE5*, and PCIE6*) *If PCIE5 or PCIE6 is occupied, PCIE4 runs at x2.

SATA SOCKETS:
K6: Total of 8x SATA3 6.0Gb/s (6x standard SATA3 and 2x ASMedia/ASM1061-controlled SATA3)
K4: Total of 6x SATA3 6.0Gb/s (standard)

M.2 SOCKET LANE SHARING with SATA SOCKETS (Both K6 and K4 have 2x M.2 sockets, the M2_1 and M2_2):
K6: M2_1 shares lanes with SATA3_0 and SATA3_1 (can only use 1)
K4: M2_1 shares lanes with SATA_5 (can only use 1)

K6: M2_2 shares lanes with SATA3_4 and SATA3_5 (can only use 1)
K4: M2_2 shares lanes with SATA_0 (can only use 1)

INTERNAL CONNECTORS (Both K6 and K4 have almost the same internal connectors, except these ones):
K6: 1x Dr. Debug with LED, 1x Power Switch with LED, 1x Reset Switch with LED, and 1x XMP Switch
K4: 1x Chassis Intruder Header

MEMORY SUPPORTED OC SPEEDS:
K6: Support up to +3866MHz (OC) RAM
K4: Support up to +3733MHz (OC) RAM

CPU POWER PHASE DESIGN:
K6: 10 Power Phase Design
K4: 8 Power Phase Design

REAR I/O PANEL (Both K6 and K4 have almost the same rear I/O connectors, except these ones):
K6: 1x USB3.1 Type-A Port, 1x USB3.1 Type-C Port, 1x USB3.0 Fatal1ty Mouse Port, 2x RJ45 LAN Ports
K4: 1x USB2.0 Type-A Port, 1x USB3.0 Type-C Port, 1x USB2.0 Fatal1ty Mouse Port, 1x RJ45 LAN Port

ACCESSORIES:
K6: Includes 1x ASRock SLI Bridge
K4: Does not include SLI Bridge (SLI is not supported)

If you plan to install an M.2 SSD, know that some SATA connectors (i.e., for your HDD, SSD, and ODD) will be disabled.
If you plan to SLI in the future, do not get the K4 as SLI is not supported.
If USB3.1 ports are important to you, do not get the K4 as it doesn't have one.
The main advantage of the K6 is its Dual BIOS support (very handy if you wish to flash/update your BIOS to latest versions).