To model the effect of the increasing load, you will need to make at least two simulations in HOMER and then do some post-processing of the results in a spreadsheet.

There are several possible approaches, but here is what I recommend:

1. Let HOMER find the optimal system configuration using the year 4 load.  

2. Simulate that optimal system configuration with the year 1 load.

3. In a spreadsheet, construct a cash flow table for the whole length of the project.  For years 1 though 3, use the fuel and O&M costs that you generated in Step 2.  For year 4 and onward, use the fuel and O&M costs that you generated in Step 1.  Calculate the total cash flow for each year, then discount all those to year zero and sum them up to find the total net present cost.

The replacement intervals will be a little tricky to deal with, since Step 1 might tell you the diesel requires replacement after 7.5 years, and Step 2 might say it requires replacement after 9.5 years.  You will have to make a few judgement calls.

You can take this approach as far as you want.  For instance, you might want to model the system in every fifth year, every third year, or even every single year of the project lifetime to account for changing load sizes or changing fuel prices.  It gets a bit much when you start considering changing the configuration of the system over time, but that is possible too.  For instance, you could start with a small diesel in year one, then several years later you could replace it or augment it with a larger diesel.