Note: iOS 4.2.1 (released in 2010) is an extremely outdated operating system. Modern apps, including Facebook Messenger, no longer support this version. This article explains the context, risks, and legacy alternatives for enthusiasts using vintage devices like the iPhone 3G or original iPad.
if you previously purchased them
Yes, but with major caveats. Apple allows downloading older compatible versions of apps with the same Apple ID. Here is the step-by-step legitimate method: