The headline rate is half the answer
Grocery bonuses are advertised as a single number, but almost all of them stop at a spending cap. A 6% card capped at $6,000 of US supermarket spend a year pays 6% on the first $6,000 and its base rate, often 1%, on everything after that.
A household spending $800 a month on food crosses $6,000 in about eight months. For the remaining four, the 6% card is one of the weakest cards in the wallet.
A 4x card with a $25,000 cap runs much longer before the same thing happens. Comparing two cards by their headline rates alone will pick the wrong one for the second half of the year.
Points are not dollars until you value them
Cash back is already money. Points are not, and the same 4x can be worth anything from a little under a cent to several cents each depending on how you redeem them.
Comparing a 6% cash back card against a 4x points card means converting both to the same unit. At a deliberately conservative 0.8 cents a point, 4x returns $3.20 per $100; 6% returns $6.00. At 2 cents a point, the 4x card returns $8.00 and wins outright.
Whichever valuation you use, use it consistently, and prefer a conservative number: an optimistic valuation flatters travel cards against cash back.
Where the bonus disappears
Supermarket bonuses are written against supermarkets. Superstores such as Target and Walmart, and warehouse clubs such as Costco, are excluded by most issuers, so groceries bought there earn the base rate.
Delivery services and meal kits are unpredictable: some code as supermarkets, some as restaurants, some as neither.
Petrol station shops, pharmacies and convenience stores keep their own category no matter what you buy inside them.