For stocks—the Four Horsemen of the Apocalypse?

Has Wall Street just met the Four Horsemen of the Apocalypse? Maybe.

1) Greece may soon exit the euro zone amid a deepening economic crisis and no relief from its creditors.

Tune in to CNBC's "Power Lunch" at 1pm ET today. Ron Insana will be on to discuss Greece and other factors affecting markets.

People wait in line to withdraw 60 euros from an ATM after Greece closed its banks on June 29, 2015 in Athens, Greece.
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People wait in line to withdraw 60 euros from an ATM after Greece closed its banks on June 29, 2015 in Athens, Greece.

2) Shanghai shares fell 3.3 percent on Monday despite another rate cut by the People's Bank of China (PBOC), and reduction in reserve requirements, which allows banks to lend more freely, and again, at a lower cost to borrowers.

3) Puerto Rico says it can't pay the $72 billion owed to its creditors and wants a moratorium on payments until it can renegotiate its external debt.

4) New York Fed President, Bill Dudley, said that a September rate hike, by the Federal Reserve, remains very much on the table, given the recent strength of domestic economic data.

The spectacle of the Greek drama is playing out, well, most dramatically in world markets, which, on average, have declined by 3 to 4 percent in Monday's trading. The exceptions are the UK and U.S., which have fallen by lesser amounts, given their relative economic stability when compared to the rest of the world.

Read MoreGreece could soon face social unrest: Wilbur Ross

So here's the question surrounding these latest events: Is any single one of these issues a death knell for the bull market in U.S. stocks?

No.

Let's take Greece to begin with. It owes its European creditors roughly $271 billion. European banks, however, have less exposure to this debt than they did during the height of the continent's sovereign-debt crisis in 2011.

Of course, if Greece defaults on its $1.77 billion debt payment to the IMF this week, it will not get any additional assistance from the European Union, nor the International Monetary Fund, which, in turn, will shift their focus toward Italy, Spain and Portugal, spending both political, and economic, capital to keep those weaker peripheral countries from suffering Greece's fate.

Read MoreGrexit is a tragedy, but 'Apocalypse Not': Strategist

There is no doubt that a "Grexit" — Greece departing the euro zone — could spark a crisis for the European economy and its single currency, the euro. But, it remains more likely that Greece will suffer the tragedy, while the remainder of the story plays out as farce.

From my vantage point, the inability of Shanghai shares to rally in the face of more easing by the PBOC stands out as a bigger issue. A crash in the Chinese market could hobble the world's second largest economy and create a different kind of contagion risk among international investors who believed Chinese stocks had nowhere to go but up. Think Japan in 1989 or the U.S. in 1999.

Puerto Rico could throw the municipal-bond market back into turmoil, while a rate hike from the Fed, even if expected here, may not play well anywhere else.

So, individually, these events wouldn't have a huge impact but taken together, they could easily create a summer of discontent for equity- and bond-market investors.

Having said that, this could also play out like 1994, 1997 or 1998, where external crises led to a friendlier Fed and extension of the economic recoveries and stock-market rallies, here at home.

Read MoreInsana: Forget Greece—it's time to worry about China

The U.S. is a bastion of stability when compared to almost any other country in the world. Any fallout should be limited, or at least one would think, given past history and past policies.

Of course, this is all predicated on whether or not central banks can get any friendlier than they already are.

With zero interest-rate policies in force in most of the world, and unconventional monetary policies also being widely deployed, friendly may not be enough this time around. So, sit tight for this week, but if China overtakes Greece as a point of concern, it may be time to protect your portfolio from the rest of the world.

Commentary by Ron Insana, a CNBC and MSNBC contributor and the author of four books on Wall Street. He is also editor of "Insana's Market Intellgence," available at Marketfy.com. Follow him on Twitter @rinsana.