Just curious, how did you test your LSD? I was under the impression it was damn difficult to quantify.
And also, since both 4-fa and LSD are stimulating the 4-fa likely wouldn't kill the trip at any dose, just the more 4-fa you add the more stimulating it will be and likely take on a distinctly different type of trip than just LSD.
I have experience with both separately, and with a few LSD+stim combos, but not this specific combo though. So take what I've said with a grain of salt.
And I don't know what m314 is saying about 4-fa lasting 4 hours. If what you have is only lasting 4 hours it's definitely not 4-fa. I had reagent tested 4-fa last 7+ hours easily. With LSD you may just want to drop the 4-fa once you feel the LSD kicking in.