It can produce the 4-PO tryptamines but it's unpredictable. In the paper Gartz published about the DET mushrooms, they produced mainly 4-HO-DET with only trace amounts of 4-PO-DET on the 1st, 3rd, 4th, and 5th flush, but the second flush had more 4-PO-DET than 4-HO-DET. I think he only got 4-HO-DPT when he used DPT. The mushrooms yielded 420mg of 4-HO-DPT from half a gram of DPT, which to me seems really impressive. The 4-HO-DiPT shrooms I ate felt just like pure 4-HO-DiPT, but then again I don't have much experience with either so I can't say anything with certainty. I don't know if they contained 4-PO-DiPT or not. I had the mushrooms GC/MS tested but no alkaloids were detected. Apparently mushrooms are hard to test.
Adding a tryptamine to the substrate will prevent the production of 4-HO-DMT. Biochemical pathways are often self-regulated by a mechanism where enzymes are inhibited by their products. So it's likely that the enzyme that decarboxylates tryptophan to produce tryptamine has an allosteric site where tryptamine acts as an inhibitor. As the concetration of tryptamine increases, the enzymes will stop producing tryptamine, limiting the production of psilocin and psilocybin. Gartz didn't detect any psilocin or psilocybin in his mushrooms, so that means that the synthetic tryptamines are able to inhibit the action of the tryptamine producing enzyme.
It's worth mentioning that I believe some of the DiPT mushrooms I had produced psilocin/psilocybin. The mushrooms were all grown the same way, yet the mushrooms from one cake had the characteristic blue color which indicates psilocybin in mushrooms, while the others were not blue at all. The non-blue ones felt exactly like 4-HO-DiPT to me, but the blue ones seemed more like regular mushrooms, with some typical mushroom visuals. That, combined with the unpredictable production of 4-PO-DET in Gartz experiment, makes it appear that the process is extremely finicky, and a lot of experimentation might be needed to achieve consistent results. But I think it could be possible to find a method that will consistently produce mushrooms high in 4-PO tryptamines.