After one-character length word or more than three-length characters word there TAB moves exactly 4 spaces. My .vimrc configuration file is it as it follows.
syntax on
" number of spaces moved along by pressing >>, << or ==
set shiftwidth=4
" number of spaces moved along by pressing the <TAB> or <BS> key
set softtabstop=4
set expandtab
The tabstop configuration value is set to default, 8. I was expecting that TAB will move forward 4 spaces no matter what.