On a MacBook forcing IPv4

NOTE: Before these change take place you need to turn off/on your WiFi. Also, these changes are persistent and will continue even after reboots.

Turn off IPv6 and for IPv4

networksetup -setv6off Wi-Fi

Enable IPv6

networksetup -setv6automatic Wi-Fi

Toggle WiFi from the command line, but use GUI instead

networksetup -setairportpower en0 off
networksetup -setairportpower en0 on