Is it ok to keep an ADULT boxer dog in a small garden for max 4 hours at a time ?

My mam will always be home so there will always be someone to look after him/her.I live near a big park , close to a beach and i will also have a lot of time to spend with the dog as i am still in school.i am considering buying a boxer pup within the next 2 months and i know i shall not keep it outside as a pup so this question is for when it gets older.i live in Ireland so the average daily temperature is 8 - 10 degrees celcius. (is there still a chance of weather related sicknesses ?).The only down side is the size of my garden.it is roughly 5 meters by 7 meters .(It this size suitable for a boxer dog from the age of 1 year and over ?) Also how is a boxer with other pets as i also have a pet rabbit. :)