The Swiss Army Knife for tax preparers is Tax Tools. One of it's nifty little tools is a Tax Fee Schedule calculator that uses IRS numbers for what a newbie grunt might spend in time to Read, Understand and Follow all of the rules and regulations to do the return correctly (Some of you may have watched Old Yankee Workshop.) Here are a few number for this return:

Form       Record Keeping     Learning           Prep/Assembly

1040        2:46                        3:58                   6:51

E              3:00                        1:13                   2:01

4797       35:23                        8:20                   9:17

So for the 4 rentals the total would be 87 hours and 52 minutes - How much should that cost this rather sophisticate taxpayer? 

You might also take into account your fee based on what does it cost you for overhead, supplies, software, education, and time spent with this couple.