I just finished breeding a Shroomish in Gen 4 that had an outstanding rating and has perfect IVs in HP, Attack, and Defense in Platinum. I also bred a competitive team of three for the Battle Frontier a few years ago with near-perfect IVs.
It’s worth noting that when you reach a certain number that Pokémon found using the PokeRadar in Sinnoh in Gen 4 have a certain number of guaranteed perfect IVs, so if you want to chain a Pokémon from the egg group of the Pokémon you want, that can really help.
I chained a Cacturne a few years ago for a shiny one, so I had one with a perfect IV in my PC, so I gave it the Power Bracer, which guarantees it passes down its Attack IV in HG/SS.
I also got lucky that the Shroomish I caught had a +Attack nature, and if you give the female an Everstone in Gen 4, they have a 50/50 chance of passing down their nature.
If you don’t care about natures, you can also give the female a power item as well and they can also pass down their best IV.
I bred about 80 Shroomish in total, every one with a guaranteed perfect Attack IV from the Poke Radar Cacturne, and in about four generations, so changing the parents with the best IVs every 20 or 30, I was able to get three perfect IVs in about two days.
If you put a Pokémon with Flame Body in the front of your party, the egg hatching goes by a lot quicker. Love to see people still playing Gen 4, hope this helps!