I recently documented some of the hardware differences between the Fold phones in a comment reply. I decided to do some additional research before cleaning up the original comment reply and making it a dedicated post. Let me know if I missed something, made a mistake, or have a question.

Keep in mind this doesn't include software related changes to One UI, camera processing, etc. nor does it include specific hardware changes that need to be tested or measured, like the display OLED materials and power efficiency, or the speakers, mics, etc.

Fold1 to Fold2

Cover display

• Size increases to 6.23", with an increase in resolution to 2260x816, change in aspect ratio to 25:9, and decrease in PPI to 386
• High brightness mode increases to 800 nits

Main display

• Size increases to 7.6", with an increase in resolution to 2208x1768, change in aspect ratio to 22.5:18, and increase in PPI to 373

• Display refresh rate is now 120 Hz, adaptive from 10-120 Hz

• Peak brightness increases to 900 nits (700 nits in high brightness mode)

SoC

• Snapdragon 865 5G+ (increase in performance and power efficiency)

Memory

• Base model storage decreases to 256 GB
• RAM is now UFS 3.1

Cameras

• Primary sensor is now the 12 MP IMX555 (no longer dual aperture but the sensor size is larger)
• Cover display camera is now 10 MP
• Ultrawide camera is now 12 MP

Hinge, dimensions, and durability

• New Hideaway Hinge now supports flex mode from 75° to 115°
• Hinge sweeper length is now 25% shorter
• 8 g heavier
• 0.7 mm thinner from the hinge when unfolded, 0.3 mm thinner from the hinge when folded
• Main display is now made of ultra thin glass, resulting in better durability, optical clarity, and feel
• Gorilla Glass Victus on the cover display

Battery

• Capacity increases to 4500 mAh
• Battery life increases due to the higher capacity and new efficient SoC

Charging

• Wired charging now supports 25 W

Connectivity

• Ultra-wideband

Fold2 to Fold3

Cover display

• Change in aspect ratio to 24.5:9 (slightly short and wider), and increase in PPI to 387
• Refresh rate increases to 120 Hz, adaptive from 60-120 Hz
• Peak brightness increases to 1500 nits (1000 nits in high brightness mode)

Main display

• Increase in PPI to 374
• Display technology is now Eco^2 OLED, resulting in a higher peak brightness of 1200 nits (900 nits in high brightness mode), and higher power efficiency
• Display now supports the S Pen stylus
• Hole punch camera replaced with an under panel camera

SoC

• Snapdragon 888 (increase in performance and decrease in power efficiency)

Cameras

• Main display under panel camera is now 4 MP (binned from 16 MP)
• Super clear glass on the rear cameras to reduce reflections and increase scratch resistance

Hinge, dimensions, and durability

• 11 g lighter
• 0.5 mm thinner from the hinge when unfolded, 0.8 mm thinner from the hinge when folded
• IPX8 water resistance rating
• Protective film is now PET resulting in a smoother feel
• Display panel layers optimized to increase durability by 80%
• Armor aluminum advertised as 10% more durable and 32% stronger

Battery

• Capacity decreases to 4400 mAh
• Battery life decreases due to the smaller capacity, and inefficient SoC, slightly offset by the new efficient display

Connectivity

• WiFi 6E
• Bluetooth 5.2
• Removed MST support for most regions

Fold3 to Fold4

Cover display

• Change in aspect ratio to 23.1:9 (slightly shorter and wider)
• Refresh rate is now adaptive from 48-120 Hz

Main display

• Change in aspect ratio to 21.6:18 (slightly shorter and wider)
• Refresh rate is now adaptive from 1-120 Hz
• Display technology is now Eco^2 OLED Plus with new organic materials, resulting in a higher peak brightness of 1300 nits (1000 nits in high brightness mode), and higher power efficiency
• Under panel camera display area perceived resolution increases by 40%

SoC

• Snapdragon 8+ Gen 1 (increase in performance and power efficiency)

Cameras

• Primary sensor is now the 50 MP GN3
• Telephoto sensor is now 10 MP with 3x optical zoom

Hinge, dimensions, and durability

• New rotational hinge, allowing for a lighter and slimmer device
• 8 g lighter
• 0.1 mm thinner from the hinge when unfolded, 0.2 mm thinner from the hinge when folded
• Fold durability of 30,000 folds at -20°C, and 150,000 folds at 65°C
• Digitizer is now reinforced with Fiber-reinforced plastic
• Panel layer material upgraded to increase durability
• New ultra thin glass advertised as 45% stronger
• Stronger adhesive on the main display protective film to prevent bubbling
• Damping structure improved with sponges to increase shock absorption
• Gorilla Glass Victus+ on the cover display and back
• Screen protector is no longer included on the cover display

Battery

• Battery life increases due to the new efficient SoC and display

Charging

• Wireless charging now supports 15 W

Connectivity

• Fully removed MST support

Fold4 to Fold5

Cover display

• Peak brightness increases to 1750 nits (1000 nits in high brightness mode)

Main display

• Peak brightness increases to 1750 nits (1200 nits in high brightness mode)

SoC

• Snapdragon 8 Gen 2 (increase in performance and power efficiency) with a 38% larger vapour chamber

Memory

• RAM is now UFS 4.0

Hinge, dimensions, and durability

• New waterdrop hinge, resulting in better durability for the main display, and allows the phone to fold completely flat
• 10 g lighter
• 0.2 mm thinner from the hinge when unfolded, 2.4 mm thinner from the hinge when folded
• Fold durability of 200,000 folds at 25°C (previous models were advertised with this but the temperature was never specified)
• Main display now has a shock dispersion layer to increase durability
• New adhesion process for the main display protective film to increase durability
• Gorilla Glass Victus 2 on the cover display and back

Battery

• Battery life increases due to the new efficient SoC

Connectivity

• Bluetooth 5.3