There are three options that I can think of off hand. The "correct" solution will depend on what works best with the data you are trying to display.
1. A tab form with a different sub-form on each page.
2. Two sub forms on the first sub-form. If they can't both be linked directly to the sub-form, you can link the "third" form to the second as you normally would and write code to synchronize the "fourth" form so that it only shows data related to the selected row of the "third" form.
3. Add a button to the "third" form to open the "fourth" form.