Also opens the door to spamming by overflowing the blockchain with data.

This is a common misconception. The 4 post limit is per account. However, nothing prevents people from creating an arbitrary number of additional accounts (potentially thousands, and there are already people with thousands of accounts) to spam. Spam control is already handled by other mechanisms which are more effective than this one.

Also, even the 4 post limit doesn't prevent spamming with a single account, it just reduces rewards. Someone who wants to be malicious or annoying can still spam.