How to toggle Vim's search highlight visibility without disabling it

Note, recent Vims (7.4.79) have the v:hlsearch variable available. This means you can improve your mapping to:

:nnoremap <silent><expr> <Leader>h (&hls && v:hlsearch ? ':nohls' : ':set hls')."\n"

" ctrl+c to toggle highlight.
let hlstate=0
nnoremap <c-c> :if (hlstate%2 == 0) \| nohlsearch \| else \| set hlsearch \| endif \| let hlstate=hlstate+1<cr>

Now just press ctrl+c to toggle highlight. Only quirk is you gotta press it twice after a search to toggle off highlight because searching doesn't increment the counter.

What I'd like is to map one key, e.g. F4, so that pressing F4 will toggle the visibility of search highlights, and so that starting a new search enables visibility no matter the current visibility.

Just tried this and seems to do the trick:

" switch higlight no matter the previous state
nmap <F4> :set hls! <cr>
" hit '/' highlights then enter search mode
nnoremap / :set hlsearch<cr>/