Counting Values Of One Column Based On The Uniqueness Of Values In Another Column

Here is a table

Col1 Col2 Col3 Col4
8821015m14:44.0E__uMail
9266321m34:13.0E__uMail
92677165m23:43.0E__uMail
9309415m07:24.0E__uMail

[Code] .....

Here is some context

Each one of these rows represents an entry into a ticket in our incident management system. Col4 represents the method by which the incident was reported to us (Email, Voicemail, Walk In, Ask IT). A ticket can be open and closed with a single entry if the issue is easily solved or it can have many entries if the issue is complex and requires more troubleshooting or escalation to a higher tier of support. I need to determine the number of unique tickets submitted by each reporting avenue (Email, Voicemail, Walk In, Ask IT).

In this table the number of tickets that were:

submitted via Email is 14
submitted via Voicemail is 1
submitted via Walk In is 2
submitted via Ask IT is 1

Col1 Col2 Col3 Col4
8821015m14:44.0E__uMail = 1
9266321m34:13.0E__uMail = 2
92677165m23:43.0E__uMail = 3
9309415m07:24.0E__uMail = 4

[Code] ....

What I need is a formula that will produce this result.