How to count number of unique values of a field in a tab-delimited text file?

Question

I have a text file with a large amount of data which is tab delimited. I want to have a look at the data such that I can see the unique values in a column. For example,

Answer 1

Assuming the data file is actually Tab separated, not space aligned:

Where $4 will be:

• $1 - Red
• $2 - Ball
• $3 - 1
• $4 - Sold