The stock market is sending retirement account balances soaring. Here's why experts say retirees still need to tread carefully.

• Stellar stock-market returns have boosted Americans' retirement account balances. 
• The number of 401(k) millionaires is up and average account balances are the highest in two years.
• Sources caution anyone thinking about retirement not to get complacent and plan ahead if they want to tap their gains. 

The soaring stock market is minting a lot of 401(k) millionaires. 

Thanks to a stunning market rally since the beginning of 2023, the number of people with at least $1 million in their retirement account jumped 20% year-over-year in fourth quarter of 2023, with that average account balance rising to the highest level in two years, according to Fidelity.