I am going to be running the D&D 4e Redbox premade adventure and a couple other premade adventures for 5 PCs. However at the beginning of the explanation for the adventures they say it is for a group of 4 PCs. Is there something simple I can do to move up the difficulty of a premade adventure to accommodate the additional PC? I really don't want it to be boring because it is too easy.
D&D 4e edition balances encounters based on XP budget
One of the joys of 4th edition is its focused on balanced, tactical combat. This is a draw for many players and to get the most out of it (fairness and challenge) appropriately balancing encounters for party size is a must. Thankfully this is very easy to do.
Each PC is worth a certain amount of XP per level (starting at 100 for lvl 1 characters). Adding another player simply means that the XP budget for the fights will go up by 100 XP (if you are starting at the lvl 1). Inclued in the monster blocktext should be an XP value, simply add a few more monsters (preferably the weaker ones) until the XP total amount for the monster side equals the PC party XP budget. Source Newbie DM
I'm going to address this question in specific context of the Redbox.
Don't adjust the difficulty until you see how the group handles the current encounters.
Counterinutivily, more PCs (unless they're experienced) really don't make encounters "too easy." Especially in the more modern adventures. They increase the tactical complexity and produce new options, but don't really chane the dynamic of the fight. I would play the adventures as-is for now. If you (for some reason) find that they're going too quickly, increase the narrative richness of combat. Amp up your descriptions of the flow of combat, the awesome arcane energies, martial feats, etc... The players are going to have enough on their plates mastering their characters and the options open to them to worry about ever more monsters.
Once you get through the redbox and its adventures, then you can get a sense for how effective your group is and tune explicitly for your group. Not all groups are created equal, and the greatest indication of challenge is system mastery. (I'd find it fairly trivial to make a two-person group that outperformed a beginner group in terms of "adventure challenge" because I whiled away entitrely too much time making characters. If they're just learning the game and not focusing on any specific optimization requirements, don't worry about monster balance at all.