Vim select the ends of multiple lines (block mode, but where the ending column varies)

1. Put your cursor on the top-left character you want to be part of the block.
2. Enter block selection mode with ctrl+v
3. 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)
4. 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.