Reverse Touchpad 4-Finger Swipe

I'm hoping this will be useful to those looking into the same problem. I was able to resolve OP's issue on my Windows 10 HP laptop with Synaptics drivers by doing the following:

Use regedit to navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Synaptics\SynTPEnh\ZoneConfig\Win10\4FHorizontal Scrolling

Swap the values for NegativeCustomZoneID and PositiveCustomZoneID. That is:
Change NegativeCustomZoneID from Hex 88 (Decimal 136) to Hex 87 (Decimal 135)
Change PositiveCustomZoneID from Hex 87 (Decimal 135) to Hex 88 (Decimal 136)

Reboot, and now the four-finger swipe direction should be successfully reversed.

In Windows 10, if you have a touchpad, you will probably have one of the options mentioned in the other answers:

Start Menu -> Settings -> Mouse & touchpad -> Reverse scrolling direction

Something manufacturer- or device-specific, probably accessible through Control Panel -> Mouse or something similar, as noted in other answers.

2nd Way: Powershell commands

Run this in PowerShell (from Start » All Programs » Accessories » Windows PowerShell):

# View registry settings
Get-ItemProperty HKLM:\SYSTEM\CurrentControlSet\Enum\HID\*\*\Device` Parameters FlipFlopWheel -EA 0

# Change registry settings
# Reverse mouse wheel scroll FlipFlopWheel = 1 
# Normal mouse wheel scroll FlipFlopWheel = 0 
Get-ItemProperty HKLM:\SYSTEM\CurrentControlSet\Enum\HID\*\*\Device` Parameters FlipFlopWheel -EA 0 | ForEach-Object { Set-ItemProperty $_.PSPath FlipFlopWheel 1 }

The command for normal (non-inverted) scrolling has the 0 and 1 swapped:

# Restore default scroll direction
Get-ItemProperty HKLM:\SYSTEM\CurrentControlSet\Enum\HID\*\*\Device` Parameters FlipFlopWheel -EA 1 | ForEach-Object { Set-ItemProperty $_.PSPath FlipFlopWheel 0 }