You're talking about valuation vs market price. There's many factors that effect both; like quality, quantity, supply and demand.

The eggs are 4700 sats. They're also $4. Merchants who define their base FX, and then subsequently trade in that, will determine it.

"we do not accept cash" - a merchant policy for example. "We only accept bitcoin" - another policy

You're not wrong in your understanding, but it's a little strained. Fx exists in any measurable way, x to y. It will always exist in a free market. If one trader has eggs for $4 CAD and another wants to sell bacon, one could equate eggs/bacon market value, attribute that to and currency and then trade it.

What you're aiming for, like many bitcoiners, is that Bitcoin becomes the standard medium of exchange/unit of exchange, as the dollar is the standard medium of exchange across base input commodities (wheat, soy, gold, rice, cobalt, diamonds, oil) all priced in USD and allows the US to export it's inflation.