Actually... technically..... 3.99 9/10 is "under" four bucks.

And therein lies your "good reason".

This is common knowledge..

Snip
“AI Overview
Pricing items at $3.99 (or other amounts ending in .99) is a traditional loss-prevention technique designed to force cashiers to use the cash register. By creating a price that requires change, it prevents a dishonest employee from simply pocketing a round-number cash payment ($4.00, in this case) from a customer.

Here is how this anti-theft mechanism works:
Forcing Register Usage: If a product costs exactly $4.00, a customer might pay with a $5 bill or four $1 bills. A dishonest cashier could easily take that cash without ringing up the sale.
Creating a Transaction Record: With a price of $3.99, the cashier must give the customer 1 cent in change. To give change, they must open the cash register drawer, which typically logs the transaction.
Theft Detection: The forced usage makes it harder to hide the transaction, as the cash in the drawer must eventually match the sales recorded on the register.

This method was popularized decades ago to combat employee theft before modern digital surveillance and computerized POS systems were common, though it remains a common practice today.”

Off Topic
Back in my High School Days my after school job was working at the restaurant where the HS kids hung out at.
My coworker and I would ring up a tab twice on the cash register ..that is if the bill was $4.75 we would first ring up $2.00 then $2.75. Each tab would count as one customer, making it appear we had a lot of customers and a busy night justifying our jobs.
The Boss never caught on.

YouTube: Happy Days theme song - lyrics on screen (Doc Doccus)

Yep, those days were just like the song. sigh..