Yes type 4 is a meeting using your personal meeting id. You can only use type 4 meeting via the web portal or client. Type 4 is when an instant meeting or scheduled meeting is using the PMI (Personal Meeting ID).
Here are the full list of meeting / webinar types:
1 Instant Meeting
2 Scheduled Meeting
3 Recurring Meeting with no fixed time
4 Meeting using Personal Meeting ID
5 Webinar
6 Recurring Webinar with no fixed time
8 Recurring Meeting with fixed time
9 Recurring Webinar with fixed time
Good question, Meeting type 7 is PAC Personal Audio Conference.
Here are the details:
Updating with more details regarding use_pmi and meeting type:
If a meeting is created with type 2 (scheduled), but with “use_pmi” option set to true, the meeting.created webhook will show type as 2. However when the host starts the meeting, the meeting.started webhook will show meeting type as 4 (because the use_pmi option was enabled).