You used to be able to put in far more than 4 items in trade. However, it was 4 items per page and devs got a lot of support emails saying they got "scammed" due to complications with there being more than 1 page in trade without it going recognized. The devs' solution was to make a maximum of 4 items in trade at a single given time. At the time it was the most number of items that could be visible in trade all at once, it prevented devs from getting annoying emails, and it was adequate for trading sets as most are 3-4 pieces.