How to paste a column of text in Notepad++ next to existing columns

You need to use the Shift + Alt shortcut to achieve your expectation.

• First the copy the content which you want to insert. (Here text4)
• Go to first line's End
• In keyboard, press Shift + Alt then also ⬇️ up to the required number of lines.
• To add space press ➡️ once (with out release the Shift + Alt)
• Now paste the copied content.

Sample execution in the GIF below:

1. Go to Replace (CTRL+H)
2. Set "Search Mode" to "Regular Expression
3. In "Find what" field enter "$"
4. In "Replace with" enter the text you want on the right side of the line
5. Click "Replace all"

The "$" symbol in regular expressions is equivalent to the end of the line and along with the Replace function can be used to "add" something to the end of the line.