After decades of working, many people envision retirement as a time when they can finally kick back and do whatever it is that their heart desires. However, in most cases, this won't be completely possible without making sure you're financially secure when that time approaches.

Unfortunately, just saving won't be enough for most people -- it's going to take investing to grow your funds. Luckily, it doesn't have to be hard or time-consuming; it just takes a solid, well-rounded portfolio to carry you throughout your career.

A well-rounded retirement doesn't require purchasing dozens or hundreds of stocks, either. It can be done with a few exchange-traded funds (ETFs) that cover a lot of ground at once. Here are four ETFs that can serve as your foundation.

1. Vanguard S&P 500 ETF

The U.S. stock market has three primary benchmarks -- the S&P 500, Nasdaq Composite, and Dow Jones Industrial Average -- but the S&P 500 is by far the most followed and important. The reason is that it tracks the 500 largest public U.S. companies by market cap, essentially making it synonymous with the U.S. economy as a whole in many investors' and economists' eyes.

While the S&P 500 is an index, different financial institutions put together their own respective S&P 500 ETFs to mirror it. There aren't many obvious differences between them, but my go-to is the Vanguard S&P 500 ETF (VOO 1.00%) because of its low cost (a 0.03% expense ratio).

Since the Vanguard S&P 500 ETF is market-cap weighted, the information technology sector makes up a large portion of it, but the fund is still fairly diverse:

  • Communication Services: 8.8%
  • Consumer Discretionary: 10.6%
  • Consumer Staples: 6.6%
  • Energy: 4.4%
  • Financials: 12.4%
  • Health Care: 13.2%
  • Industrials: 8.4%
  • Information Technology: 28.2%
  • Materials: 2.5%
  • Real Estate: 2.5%
  • Utilities: 2.4%

The S&P 500 is a trifecta: It's diversified, contains every blue chip company, and has a proven track record of returning good value.

2. Vanguard Russell 2000 ETF

What the S&P 500 is to large-cap stocks, the Russell 2000 is to small-cap stocks. It's an index tracking the 2,000 smallest stocks in the Russell 3000. Small-cap stocks generally have a market cap between $300 million and $2 billion, and it's because of this relatively small size that they can be a gift and a curse.

On one hand, being small means there's lots of room for growth, which typically means lots of money to be made for investors. It also means these companies are more prone to volatility and broader economic conditions, adding more risk than larger companies with many more resources.

Since small-cap stocks are high-risk, high-reward, you don't want them taking up too much of your portfolio, but you still want to give yourself a chance to experience some of the opportunities that can come with them. The Vanguard Russell 2000 ETF (VTWO 0.97%) is a good ETF choice for this.

3. Vanguard Mid-Cap ETF

Mid-cap stocks are the sweet spot between large-cap and small-cap stocks. These are companies with a market cap between $2 billion and $10 billion, meaning they're just big enough to have established operations and stability, but also small enough to have high growth potential.

Having a portfolio of two extremes -- large-cap and small-cap -- isn't the ideal choice for a well-rounded portfolio; you also want exposure to companies that operate in the middle. Plenty of mid-cap ETFs contain companies from every sector with a track record of providing good results.

Take the Vanguard Mid-Cap ETF (VO 0.24%), which contains 340 stocks. The holdings are also fairly spread out, with the top five sectors represented being:

  • Consumer Discretionary: 13.5%
  • Financials: 12.3%
  • Health Care: 10.5%
  • Industrials: 17.8%
  • Technology: 14.3%

The Vanguard Mid-Cap ETF also has the bonus of an above-average higher dividend.

4. Vanguard Total International Stock ETF

True diversification and well-roundedness requires investing in companies outside of the U.S.

International markets are generally labeled as developed or emerging. Developed markets have more mature financial markets and infrastructure (think: the U.S., Australia, and Japan), and emerging markets are seen as progressing toward that (think: Mexico, Brazil, and India).

An ETF like the Vanguard Total International Stock ETF (VXUS 0.81%) is a great option because it contains over 7,900 companies from both markets:

  • Emerging Markets: 25.1%
  • Europe: 40.4%
  • Pacific: 26.9%
  • Middle East: 0.4%
  • North America: 7.2%

Having companies from developed and emerging markets gives investors the relative stability and reliability that come with developed markets, as well as the growth potential that comes from emerging markets as they ramp up development.

The Vanguard Total International Stock ETF also has a dividend yield of around 3.1%, roughly double that of the S&P 500. It can be a solid and stable source of income for investors.