Agree with the others on the primary direction & control question. This is important to verify. If they don't meet the primary direction & control requirement, then giving them a contribution in your client's plan could violate the exclusive benefit rules. Also, if they receive a 3% contribution in your client's safe harbor plan, shouldn't they also have the option to make deferrals to that plan?
You might look further into the PEO rules. Based on the facts presented, it sounds like the leasing org might really be a PEO. If that's the case, the terms of their multiple employer plan may affect your client's options. Check out Rev. Proc. 2002-21 & 2003-86 and cross your fingers they have a multiple employer plan if they are indeed a PEO.
If it's truly a leasing situation, has your client considered the leased employee safe harbor - 10% money purchase plan with immediate participation/vesting - which eliminates the much of mess for your client since the 4 employees wouldn't have to be counted on the practice's nondiscrimination testing.