^ Definitely a bad idea, especially such a combination. Just because you've pushed high doses and gotten away with it does NOT mean those doses are safe.
The fact with the NBOMes is that they cause extreme amounts of vasoconstriction - the reason this is dangerous is that as your blood vessels are constricted, your blood pressure increases. To put it in perspective, 1.2mg of 25C-NBOMe buccally increased my blood pressure significantly more than 3-4 days BINGING on the super strong stimulant MDPV. It wasn't quite to the level of "You should go to ER right now" but how fast it rose to such a level makes me think even the smallest dose increase would certainly put someone in the danger zone.
25C-NBOMe seems to be significantly worse for this than 25I-NBOMe, but both, and the other NBOMes all have this risk.
I can certainly agree with why you might not find it intense enough, for example I wouldn't push higher than the 1.2mg of 25C-NBOMe I've done simply because of how the vasoconstriction was already approaching dangerous levels - but in many ways the experience felt lacking and like I certainly wanted something much stronger.
The fact is though, these simply aren't the drugs for that - these, like the other notoriously vasoconstricting psychedelic, Bromo-DragonFLY, do NOT have the safety profile of other psychedelics, and yeah you might get away with pushing high doses a handful of times but that doesn't mean you wont end up in hospital or dead next time.
These simply aren't as safe as other psychedelics at high doses, so if you're looking for a crazy super intense experience, look to other safer drugs where the dose range can be pushed.
Also, while I didn't find 1.2mg of 25C to be quite enough for example, your doses still seem ridiculously high. I suspect either your blotter is not as strong as you've been told or you are swallowing the blotters too quickly when dosing buccally for example - I'd recommend holding them 60+ minutes, any less and you're wasting a significant amount of material really, since these chemicals are not orally active. If you've been holding them for say 10-20 minutes in the past then that could explain why you've needed to (and been able to) push the dose so high, and if this is the case I'd recommend dropping your dose to at least half the previous dose before increasing how long you hold them.
I would go as far as saying that the blotters are likely weak though. I typically need slightly higher doses of psychedelics than most to reach the same level of intensity, but 1mg of 25I-NBOMe was almost too much. With 25C-NBOMe the problem was the intensity seemed limited to the come up and the trip itself felt very mild afterwards so I could see wanting a higher dose, but needing 4mg of 25I? Either your tolerance is already incredibly high from other drugs, or those aren't the kind of dosages you think you're taking necessarily.
How long did you space your trips out? You should remember that between NBOMe trips you need at least 2 weeks really to significantly lower the tolerance, ideally 3-4+ particularly if you've done high doses or tripped consecutively. If you haven't been waiting this long then this would explain why you've been able to go from low doses to such high ones and not reach the intensity you're looking for.
The other problem is these drugs are lacking in a certain area that adds to the intensity of psychedelic experiences, I noticed that even when my vision can be overwhelmed with visuals to the point I can't see in front of me, I still feel completely in control and almost "sober" at points. I don't think this is going to change at higher doses, as certain psychedelics are simply like this.