I need to add 3 if not 4 time zones to my Google Calendar, but I do not see the option for this.
What would be the way of achieving this?
At this time there are only two time zones allowed. The option open to you is to suggest to Google adding more than two displayed time zones.
The best place to check first is the official suggested features page for Google Calendar. Seeing as how I could not find this suggestion, it is likely not a high priority for the Google Calendar team at this time.
Another rout to suggest this feature is by going to the Scheduling across time zones Google help page. There is a rating at the bottom of the page "How helpful is this article:" where I would select "Not very helpful" this will bring up a text box where you can leave your feedback. I would place your request here.
Sounds like you have already done this, but to add multiple time zones the steps are below:
Go to your Google calendar > select settings (gear icon) > under the General tab their is a field called Your current time zone, select Show an additional time zone.