Depending on what you mean by tab, one of the following should work:

" size of a hard tabstop

set tabstop=4

" size of an "indent"

set shiftwidth=4

" a combination of spaces and tabs are used to simulate tab stops at a width

" other than the (hard)tabstop

set softtabstop=4

You may also want to try the following:

" make "tab" insert indents instead of tabs at the beginning of a line

set smarttab

" always uses spaces instead of tab characters

set expandtab

See :help 'optionname' (eg: :help 'tabstop') for more details on any of these.

tl;dr

In a codebase that uses 4 space characters for each indent, here are good settings to start with:

set tabstop=8 softtabstop=0 expandtab shiftwidth=4 smarttab

In a codebase that uses a single tab character that appears 4-spaces-wide for each indent these settings should work:

set tabstop=4 softtabstop=0 noexpandtab shiftwidth=4