You can use 401k or stocks account.

@Costin IorgulescuAs I noted, if investing in rental property will produce better results in terms of risk/reward than continuing to invest in the stock market only, then it is the right move.There is no apples-to-apples comparison between you personally investing in real estate and your IRA investing in real estate, just as comparing you investing in stocks with personal funds would also be very different from you investing in stocks with your IRA. 

You might also try someone like TD Ameritrade or Schwab which is really an investment account to trade stocks, but they typically offer some banking services as well. 

Suppose the worst case scenario transpires, can you borrow from a 401k or credit card at a reasonable interest rate, sell some stock, etc.?

This also happens with stocks.

Scenario A (typical) - $500k duplex-$125k down-$375k mortgage at 4.6%Scenario B (modular financing) - $500k duplex-$125k down-$260k mortgage at 4.6%-$75k HELOC on primary residence-$40k loan against 401(k) (technically this would be $165k down, but you get the point)In scenario A, paying off the mortgage quickly makes zero improvement on cashflow until you pay it off completely, or refinance, and there's no point in that if your rate is locked in lower than current(or future) market rates.Scenario B could involve higher interest rates on the HELOC and the 401k loan, but you have multiple, simple, easy options for increasing your cashflow, and then you don't end up playing as much in the overpriced, volatile stock market.

I have the stock options, great retirement, and pretty good pay. i make about 29k-30k a year right now and I'm 20 years old.

We own a house, and I solely manage our stocks/options portfolio.So far I like the idea of REI, but my first hurdle will be convincing my wife.

focus on your career/work/training/primary income and also (if applicable to you), take stock on any bad spending habits.  

Same thing if you "borrow" against your stocks and they lose value and/or rates continue to go up.