Lightsail with 4 Wordpress sites stalling with high CPU load?

Lightsail instances comes with a baseline amount of CPU performance, but also have the ability to temporarily provide additional CPU. This is referred to as bursting.

As per documentation, The Linux or Unix .... (2 vCPUs, 2 GB memory, 60 GB storage) instance plans include a 20% CPU utilization performance baseline.

Your instance will use all of its burst capacity if it operates in the bursting zone frequently, or for extended periods of time. This can signify that your instance is under-provisioned. It could also be that a service is running too frequently, or your instance is running unnecessary software.

You may want to Troubleshoot high CPU utilization

Hello.

The Lightsail plan you are using starts to burst at CPU usage above 20%, so I believe that if you host 4 websites within one instance, CPU usage will easily exceed 20%.
Therefore, it may be necessary to take measures such as dividing Lightsail instances for each website or migrating to EC2 etc., although the cost will increase.
https://docs.aws.amazon.com/lightsail/latest/userguide/amazon-lightsail-viewing-instance-burst-capacity.html