If Harmalas for you only lasts 4 hours regardless of amount taken, then that to me says you're likely higher in CYP2D6 compared to the average person, which would not only require more Harmalas for an equivilant dose/effect compared to what the average person may require using a bit of a lower dosage, and would also shorten the Harmala duration which would move up the timing for the gut's MAO-A inhibition, which i'm not sure of the timing between Harmalas and DMT for those with high CYP2D6, i'm not sure of the timeframe and how much shorter the window may be, so it'll take a little experimentation taking the DMT say at the same time as the Harmalas, vs 10 minutes, 20 minutes, 30 minutes, 40 to 45 minutes, an hour etc, and find the best time for you when you feel that the gut's MAO-A enzyme is more fully inhibited.
For those normal/average in CYP2D6 (which i believe i am), taking the DMT an hour into the Harmalas seems best ime. Those lower in CYP2D6 would need less Harmalas for a strong dose and it would last them longer and would be more stretched out.
Speaking of which, if you are higher in CYP2D6, you could try taking a dose of Harmalas prior to the main dose of Harmalas, so like take a dose (could be a low dose, moderate, high), and say anywhere from an hour to a few hours in try taking another dose of the Harmalas, which should then be potentiated by the previous Harmala dose due to it's CYP2D6 inhibition and would then not only increase the bioavailability but also the duration, if you catch the timing properly for the CYP2D6 inhibition. But again, anything that moderately to strongly inhibits CYP2D6 (so certain medications or supplements or herbals for example) could be used as well for that purpose imo, in order to potentiate the Harmalas dosage-wise and duration-wise so that you can get a longer experience, if desired. For me Harmalas themselves last 6 to 8 hours, up to 10 to 12 hours with higher dosages especially as the reverse tolerance builds up, for the DMT it always lasts 4 to 5 hours for me, any less than that and something somewhere went wrong lol. So overall for me this stuff lasts in general about 8 hours in total for the most part but i'm usually in the afterglow around the 6th to 7th hour.