If you're having trouble connecting to the Call of Duty: Black Ops 4, you should first check Call of Duty: Black Ops 4 Server Status.
If the status indicator for your platform is not green or an alert is present, you might encounter connectivity issues until service is restored and fully operational.
However, if the status indicator is green and no alerts are posted, several other factors could be affecting the quality of your connection.
"We strongly recommend using a wired Ethernet connection. A wired connection will allow your system to achieve optimal performance when playing an online game. Wi-Fi connections have a wide variance in reliability and bandwidth, and can dramatically affect the quality of online gameplay. Wired Ethernet connections will maintain the minimum bandwidth connection that is required for online gaming."