Mistaken idea of what a shamrock looks like. Both the oxalis and shamrock have 3 leaves, not 4. It is a clover, genus trifolium.
Yes, especially St Patrick's, the whole point of which was the it was suppossed to be a metaphor for the holy trinity.
To show stupid pagans, what a really clever idea it was, and how much smarter Christianity could be. (Irony)
No you had it right the first time, you should not smoke clovers.