It’s a pretty big rule – in life, and in marketing, and it’s about establishing trust.
Do what you say you’re going to do.
Your words are your marketing.
If you offer to deliver 4 items, provide 4 and delight people with another one.
Don’t leave them wondering where the 4 are and whether they are ever going to get them.
Also, there’s no point delighting them with an extra, if they didn’t get the 4 you promised.