The past decade has been enormously rewarding for stock investors. Even after the recent drop, which still has stocks down almost 15% from the peak in early October, the S&P 500 has delivered an incredible 244% in total returns over the past 10 years. Here's the thing: While it has been a great decade, it's actually relatively normal for the stock market to deliver big gains over a 10-year period. 

Simply put, stocks have proven, time and again, to be the best, most-accessible tool for the average person to build long-term wealth with. 

Furthermore, the recent sell-off has created some excellent opportunities to buy top-notch stocks at big markdowns. This includes two leading financial growth stocks, Axos Financial Inc. (AX 1.52%) and Square Inc. (SQ 0.15%), as well as two top-notch infrastructure and energy stocks, NV5 Global Inc. (NVEE 0.92%) and TerraForm Power Inc. (TERP)

Five hands reach up for a dollar bill.

It's not a reach to say these four stocks could play big parts in helping create the next generation of millionaire retirees. Image source: Getty Images.

All four trade for big discounts to their recent stock price peaks, with three down more than 30% at recent prices despite having excellent long-term prospects. Keep reading to learn why now is an excellent time to invest in these potential millionaire-making stocks. 

Two big-growth financial stocks on sale

As of this writing, shares of both Axos Financial and Square are down more than 40% over the past six months. And while it's arguable that both stocks may have gotten a bit overheated earlier this year, I think there's also a solid argument that they're oversold at this point, and represent excellent buys at current prices. 

Let's start with Axos Financial. Formerly known as BofI Holding, Axos has grown from being primarily a Southern California jumbo mortgage lender and online bank to a full-service commercial bank. Home loans are still its bread and butter (as is the case for most banks), but expansion of its other lending lines and services has pushed residential mortgages below 50% of its lending book. This diversification of its lending, as well as the growth of its other banking services, should continue to serve it well in terms of delivering continued growth. 

Lastly, Axos is just dirt cheap. At recent prices it trades for 10.6 times trailing earnings per share, and 1.6 times book value. I've already called it my best growth stock idea for 2019, and it's 10% cheaper today. With multiple decades of growth ahead, and one of the best CEOs in banking calling the shots, Axos is a "buy now" growth stock that should deliver years of market-smashing returns. 

Square's stock is also down more than 40%, but I'll hesitate to call it a value stock because it has yet to generate a GAAP -- generally accepted accounting principles -- profit as a public company. But it has steadily proven it can generate positive operating cash flows -- a decent proxy for sustainable operations, especially for a high-growth company like Square. Furthermore, those cash flows are growing at a very high rate, as is the rate it is cutting its losses and moving toward GAAP profitability.

While it's not exactly a value, the price-to-potential ratio is very, very good. Few companies are growing anywhere close to as quickly as Square, and its business is so compelling, one of my colleagues named it his "one stock to buy" this year. To sum it up, Square is growing revenue over 50% per year and growing its cash flow at a strong clip, but its stock is down 40%. That's a wealth-building opportunity if I've ever seen one. 

Building the future

Between aging infrastructure and the growth rate of the world's urban middle class, nearly $90 trillion will need to be spent on infrastructure modernization and expansion over the next few decades, and one small U.S. engineering and consulting company, NV5 Global, is already taking more market share from its bigger competitors. 

With trailing revenue of less than $400 million, NV5 Global is a veritable drop in the infrastructure ocean, but it's growing quickly through both acquisitions and organic expansion, taking a steadily bigger bite out of the global pie. Over the past year, NV5 has grown revenue by 19%, and earnings almost 25% per share. 

And that pie is enormous, giving the company a potentially very bright future as high-growth countries add more infrastructure to support their burgeoning middle-class populations and rapidly expanding economies. Furthermore, slower-growth economies like the U.S. are faced with aging infrastructure that must improve to maintain quality of life for its citizens and to remain relevant and competitive for business and industry. 

After the big sell-off, NV5 shares trade for less than 22 times trailing earnings and less than 16 times projected 2019 earnings. Compare that to the 19 times earnings the S&P 500 trades for at recent prices and you have a very fair price for a company that's growing earnings at over double the rate of the market -- and with the potential to keep it up for many years to come. 

Powering the future

Many of the same macro things boosting infrastructure needs are a big tailwind for renewable energy. Aging coal and nuclear power plants will need to be retired and replaced in the decades ahead, and technological improvements point toward wind and solar replacing much of it. Furthermore, global middle-class growth means global power production will have to expand, and cheap wind and solar will be major sources of this growth. 

TerraForm Power is well positioned to be a huge winner from this trend. TerraForm has transitioned from a top-heavy company that failed to deliver good returns to a far more efficient energy producer that's turning more and more of its incoming revenues into profitable cash flows.

With a dividend yield of 6.8% at recent prices and a cash flow trajectory that makes that payout sustainable and likely to increase in coming years, it's already a solid income investment. But the global demand for more energy -- and the reality that wind and solar are becoming global low-cost sources -- sets it up for potentially decades of growth ahead. Investors who buy TerraForm Power today and hold for the long term should be able to count on it as a key part of a millionaire-maker retirement portfolio.