You've raised a good point about the discrepancy between the general demand factors and the specific allowance for ranges. Let's break this down

Understanding the Code Sections
220.84 (Optional Method - Multifamily Dwelling)

This section provides a streamlined approach to calculating the service load for a multifamily dwelling. It uses demand factors based on the total square footage of the units and the number of appliances. The 45% you mentioned does account for some diversity in appliance usage, but it's a broader, less granular adjustment.

220.55 (Ranges)

This section specifically addresses the load calculation for electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances individually rated more than 1.75 kW. It allows for a significant reduction in the calculated load because, as you correctly point out, it's very unlikely all elements of a range will be at full power simultaneously.

Your Specific Situation

You are right to question if you can use 220.55 after you use 220.
84. The answer is generally no, you cannot mix and match these sections in this way. Using 220.84 locks you into that method. This optional calculation method is intended to simplify the process. If you choose to use 220.84, you must apply all the factors within that section. You cannot then cherry-pick a more favorable provision from another section. 220.80(A) states that the optional methods are permitted as an alternative to Part III for dwelling units. Applying 220.55 before using 220.84 is possible, but it may not be the most advantageous approach. If you were to calculate the load for each individual unit using the standard method in Part III and then add those loads together, you could use 220.55 for each range. However, this is more work and, in some cases, 220.84 might still result in a lower calculated load.

Why the Discrepancy Exists

The optional calculation method (220.84) uses broader averages and statistical data to simplify the load calculation for larger buildings. It assumes a certain level of diversity in appliance usage across all units, which is reflected in its demand factors. However, it's less precise than calculating each unit individually and applying the more specific range demand factors.

The Best Approach for You (Generally)

Since you've already used 220.84, you are locked into using the parameters set by that section and cannot apply 220.
55.

However, let's look at the impact of a full 15kW calculation

If you have already calculated the load for all four units using 220.84 and included the full 15kW range load for each unit, you might be able to argue that the overall demand factor you used already provides sufficient diversity. You can also try to figure out what each range calculates to using 220.55 and then use the standard calculation to see if you can get away with a slightly smaller service than the 220.84 method.

Important Considerations and Recommendations Local Inspector Authority

Always consult with your local electrical inspector. They have the final say on whether a particular installation complies with the code. They may be willing to consider your arguments about diversity and the improbability of all range elements operating at full power.

Future Load Growth

While you might be fine with the calculated load now, consider potential future load growth. Will tenants be able to install electric vehicle chargers, add window air conditioners, or other high-demand appliances without overloading the system?

In summary: While it's tempting to use 220.55 to reduce the range load calculation, you are generally required to adhere to the chosen calculation method (220.84 in your case). The 45% demand factor in 220.84 accounts for some appliance diversity. Consult your local inspector and consider future load growth before making your final decision. Flag for review