If you use the Citi Visa and expect to get 4% back on gas, this is not actual if you get gas from another grocery store. You only get 4% back if you purchase gas from a stand alone station. If you buy from, Safeway, Kroger, Meijer etc… the cash back is only 1%. (Any competitor)
I also fill up at Safeway so I checked my balance and it was not what I thought so I called Citi and they explained that any “competitor” is excluded from the 4%. After much searching on their website I finally found the exclusions (its a long list)
I believe Costco should be more forth-coming on their policy or at least make it easy to find so that the consumer is aware of this. 
Below is what I FINALLY found on the site
Earning Cash Back Rewards on Purchases: You’ll earn cash back rewards for purchases using your Card Account, minus returns and refunds, as follows:
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5% cash back on gas purchased at Costco warehouse locations worldwide and 4% cash back on other eligible gas and eligible electric vehicle (EV) charging purchases worldwide for the first $7,000 per year in combined Costco and other eligible gas and/or EV charging purchases, and then 1% thereafter.
- Certain Non-Qualifying Purchases. You will only earn 1% cash back, not 5% or 4%, for gas or EV charging purchased at some superstores, supermarkets and warehouse clubs other than Costco or for fuel used for non-automobile purposes. For a list of merchants excluded from earning 4% for gas or EV charging purchases, visit citi.com/Costco_Exclusions.