Can we or should we do a Joint Venture or Syndication attached to our LLC or should we start a fresh deal between the 4 of us?

Rent after this is one check and the lease says they are jointly and severely liable for rent.

The exception would be a qualified joint venture. 

@Tony Marcelle - There are experienced note investors who look for "funding partners" to join them in a Joint Venture where they provide access to assets and expertise in exchange for a share of the profits - usually around 50%.

If they pay with a check or ACH, you wait until the payment clears.

We decided that instead of structuring this as a joint venture we wanted it to be set up so that he is the private investor giving me a loan to fund the flip and fix so when it is finished I will pay him back his investment plus interest and the remaining profit will go to my LLC. 

There is a fish joint right next door.

I believe he did a Joint Venture on a investment property.

For taxpayers in the 25% - 35% bracket the rate is 15%For taxpayers in the 39% bracket the rate is 20%For most capital gains the net investment income tax of 3.8% kicks in on the lesser of the capital gain or the excess of their AGI over $250k (married) and $200k (single).For example, $1 million capital gain, married filing joint, no other income: Capital gain up to $75k (roughly) taxed at 0% Capital gain of $390k (roughly the 25% - %35% bracket range) taxed at 15% = $58,500 Capital gain of $535 taxed at 20% = $107,000 Net Investment Income Tax ($750k @ 3.8%) = $28,500 Total Tax = $194,000 Plus state taxThere are other variables but this is the basic idea.

You need to secure your investment with either a notice of interest filed against the property (at minimum), to even get repaid, or a 2nd mortgage, or a joint venture with the first private lender and a new LLC to lend to the borrower in 1st position.