if you're filtering using peat and starting with RO water, then dropping the pH that low should be a piece of cake. *in a separate container* add some hydrochloric acid (miniscule amounts will do... one drop if its a solution, or the tiniest of tiny pinches if it's a solid. you might just have to double or triple the volume of water vs. whats in your tank to make tweaking the values easier. I have a few questions for you, though.
1. pair of what? i'm not aware of any fish commonly kept in the hobby that *require* a pH that low to spawn. *most* blackwater fish will spawn in a pH of 5 - 7.0, and from what I understand, pH isn't as important as conductivity, or TDS.
2. are you injecting CO2 into this setup? if not, then I'd just leave the CO2 chart out of it for now, just confusing the issue for no real reason. if you are injecting CO2, then reaching a pH that low shouldn't be very hard with the water source you're using. water with no kh, like RO, distilled, rainwater, etc., has nothing in it to neutralize acids added to the water (HCl, CO2, etc.), and will drop VERY QUICKLY with the addition of acids to the water.
3. how do you plan to test a pH this low? do you have a special kit or an electronic meter? anything below 6 would be tough to keep track of with most commercially available kits / strips. (there are low range ph kits from specialty sources, and would serve you well if you don't already have something on hand).
I'm interested to hear what you're keeping in a tank that small that likes a ph of 4-4.5. If it's apistos, then the tank is probably pretty small for them. I hope some of this info is helpful to you in your endeavors to breed your mystery pair
keep us updated on progress, and methods used so that others can duplicate your success and learn from your experience. Best of luck!