- Put your cursor on the top-left character you want to be part of the block.
- Enter block selection mode with ctrl+v
- Select to the end of the line with $ (this is the step you're missing; if you move to the end of the first line using $ then the selection will extend to the end of subsequent lines as well)
- Move down 3 lines with 3j
There's more information in the Vim documentation's section on visual mode which you can read online, or just type :help v_$
in Vim.
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