@Bin Chen, If handled correctly the property  was contributed  to the LLC at basis so the 400K gain will be recognized by the LLC and then passed to the two of you as members through the partnership return.Your opportunity is that any tax filing entity can complete a 1031 exchange.  

My wife tells me I never saw you work so hard and I told her, you know, that's the difference....I don't feel I'm working when I'm on BP contributing to others and learning from others.  

If you need to add money to the account you just pay into their system, via an owner contribution to cover the cost of the bills.

That down payment is your capital contribution.

In the podcast, it was said that a person can contribute up to $50,000/ year if you're self-employed, but only $5,000 if you're a W2 employee.  

Sounds like time is a real factor.Consider refinancing your existing properties, leverage them to acquire more, you really need to examine this closely, finance terms, your cash flow and how you use your money.I suggest you not partner, generally such deals can take more time, dealing with partners and tenants and your return is split, it can be valuable for younger beginners who can contribute more time and effort with limited funds.You may also consider seller financed deals, you can pledge the equity of one property (as a second) to leverage the next deal, you give some cash and a note to your seller. 

 Check with your plan sponsor but if you take a loan from your 401k you may not be able to contribute new funds until it is paid back.

You may also want to look at the solo 401k plan.Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k Similarities Both were created by congress for individuals to save for retirement;Both may be invested in alternative investments such as real estate, precious metals tax liens, promissory notes, private company shares, and stocks and mutual funds, to name a few;Both allow for Roth contributions;Both are subject to prohibited transaction rules;Both are subject to federal taxes at time of distribution;Both allow for checkbook control for placing alternative investments;Both may be invested in annuities;Both are protected from creditors;Both allow for nondeductible contributions; andBoth are prohibited from investing in assets listed under I.R.C. 408(m).The Self-Directed IRA and Solo 401k DifferencesIn order to open a solo 401k, self-employment, whether on a part-time or full-time basis, is required;To open a self-directed IRA, self-employment income is not required;In order to gain IRA checkbook control over the self-directed IRA funds, a limited liability company (IRA LLC) must be utilized;The solo 401k allows for checkbook control from the onset;The solo 401k allows for personal loan known as a solo 401k loan;It is prohibited to borrow from your IRA;The Solo 401k may be invested in life insurance;The self-directed IRA may not be invested in life insurance;The solo 401k allow for high contribution amounts (for 2016, the solo 401k contribution limit is $53,000, whereas the self-directed IRA contribution limit is $5,500);The solo 401k business owner can serve as trustee of the solo 401k;The self-directed IRA participant/owner may not serve as trustee or custodian of her IRA; instead, a trust company or bank institution is required;When distributions commence from the solo 401k a mandatory 20% of federal taxes must be withheld from each distribution and submitted electronically to the IRS by the 15th of the month following the date of each distribution;Rollovers and/or transfers from IRAs or qualified plans (e.g., former employer 401k) to a solo 401k are not reported on Form 5498, but rather on Form 5500-EZ, but only if the air market value of the solo 401k exceeds $250K as of the end of the plan year (generally 12/31);When funds are rolled over or transferred from an IRA or 401k to a self-directed IRA, the amount deposited into the self-directed IRA is reported on Form 5498 by the receiving self-directed IRA custodian by May of the year following the rollover/transfer.Rollovers (provided the 60 day rollover window is satisfied) from an IRA to a Solo 401k or self-directed IRA are reported on lines 15a and 15b of Form 1040;Pre-tax IRA contributions on reported on line 32 of Form 1040;Pre-tax solo 401k contributions are reported on line 28 of Form 1040;Roth solo 401k funds are subject to RMDs;A Roth 401k may be transferred to a Roth IRA (Note that from a planning perspective, it may be advantageous to transfer Roth Solo 401k funds to a Roth IRA before turning age 70 ½ in order to escape the Roth RMD requirement applicable to Roth 401k contributions including Roth Solo 401k contributions and earnings.)

What's your take on partnering to have more contribution to down payment and possibly acquiring more units?

Lastly, I wouldn't have really got this far without everyone from BiggerPockets, thank you for all the interesting and inspiring articles and posts, I hope that my contribution might inspire someone else to take the next step. ~ Caleb