http://www.mangago.me/read-manga/nijiiro_days_mizuno_minami/
this is not a one shot XD
MrsHatake was right, these 4 boys indeed are from Nijiiro Days. Idk if I understand you, do you mean you are or aren't looking for an oneshot? There's an one-shot included in the manga like a prequel story, written as ch. 0: http://www.mangago.me/read-manga/nijiiro_days_mizuno_minami/mh/v01/c000/pg-1/
Can you help me find this manga, I've read this before but for some reason it's not on my list
http://www.mangago.me/read-manga/kimi_ni_koishitei_desu_ka/mh/c001/2/