Here's a tip to help you set up A4 sized posters easily. After printing my posters, I skip laminating and just insert them into vinyl report covers (with the clips). This way, the posters can be seen clearly and are protected by the glossy transparent cover. The best part about it is that you can change the posters easily by just taking out the page and putting in a new one. While the vinyl covers will remain on the wall all year, the posters inside can be changed as often as needed.
I applied some heavy duty mounting tape at the back of each, and placed a pair of adhesive-backed magnetic dots on the inside of the top corners to keep the vinyl covers closed. You can always use Velcro dots instead of the magnetic dots if you prefer because the Velcro dots are stronger. I chose the magnetic ones because they always give a sleeker finish.
Currently, I am using them for my grammar and vocabulary display areas. I got those amazing (and free) posters from Corinna at Surfing Through Second. Feel free to grab your copy there! Here are links to free downloads of other things you may need to have on display: monthly calendars, class rules, motivational posters, math posters, reading posters, etc. I am really pleased with how this idea turned out so I'd love for you to give it a try sometime!