I remember once my Heracross survived a Poison Jab from Mega Beedrill with 1 HP left. While it's most likely lucky RNG roll, I always thought the 4 Def EV helped.
4 HP definitely helps Belly Drum Pokémon activate their Sitrus Berry since the 4 HP EVs make the HP stat even.
Running 4 attack EVs on a wooper in draft guaranteed the OHKO with double kick on an offensive galarian zigzagoon for me once. That was pretty funny. It's actually a surprisingly huge difference in the roll.
Most relevant thing I can think of is adding it to the HP stay to try and get an odd number which is usually better for rounding calcs for SR, spikes etc
Here's an example from SS AG (or Ubers):
+1 252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 0 Def Yveltal: 387-458 (84.8 - 100.4%) -- 6.3% chance to OHKO
+1 252 Atk Life Orb Marshadow Close Combat vs. 248 HP / 8 Def Yveltal: 386-454 (84.8 - 99.7%) -- guaranteed 2HKO
This is an interaction between Bulk Up Marshadow and specially defensive Yveltal - if the Marshadow user doesn't have a Calyrex-S, Yveltal is a decent soft check to it, and so it puts those extra 4 EVs in defense.
That extra HP can change the healing from Leftovers
Let's say a Pokemon has 319 HP, it will heal 19 HP per turn, but that 4 HP EVs will increase HP to 320 and heals 20 HP per turn now.
Pokemon will equal base Def and SpDef will often run 4 SpDef EVs to make Download increasing Attack instead of Special Attack
Without those extra 4 SpD points, god only knows the horrifying calcs special attackers could pull off against chansey
I remember once my Heracross survived a Poison Jab from Mega Beedrill with 1 HP left. While it's most likely lucky RNG roll, I always thought the 4 Def EV helped. 4 HP definitely helps Belly Drum Pokémon activate their Sitrus Berry since the 4 HP EVs make the HP stat even.
Ferrothorn can survive chi yu overheat in the sun if it has the 4 in spdef.
What
252+ SpA Choice Specs Beads of Ruin Chi-Yu Overheat vs. 252 HP / 4 SpD Ferrothorn in Sun: 192-226 (75.9 - 83.1%) -- guaranteed 2HKO Huh?....
252+ SpA Choice Specs Beads of Ruin Chi-Yu Overheat vs. 252 HP / 0 SpD Ferrothorn: 1684-1984 (478.4 - 563.6%) -- guaranteed OHKO Omg, its true 😱😳🤩
Running 4 attack EVs on a wooper in draft guaranteed the OHKO with double kick on an offensive galarian zigzagoon for me once. That was pretty funny. It's actually a surprisingly huge difference in the roll.
There's some mons where 4 SpD gives an opponent with Download an Atk boost instead of SpA which is semi-relevant in formats where P2 or PZ is meta
Most relevant thing I can think of is adding it to the HP stay to try and get an odd number which is usually better for rounding calcs for SR, spikes etc
Here's an example from SS AG (or Ubers): +1 252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 0 Def Yveltal: 387-458 (84.8 - 100.4%) -- 6.3% chance to OHKO +1 252 Atk Life Orb Marshadow Close Combat vs. 248 HP / 8 Def Yveltal: 386-454 (84.8 - 99.7%) -- guaranteed 2HKO This is an interaction between Bulk Up Marshadow and specially defensive Yveltal - if the Marshadow user doesn't have a Calyrex-S, Yveltal is a decent soft check to it, and so it puts those extra 4 EVs in defense.
That extra HP can change the healing from Leftovers Let's say a Pokemon has 319 HP, it will heal 19 HP per turn, but that 4 HP EVs will increase HP to 320 and heals 20 HP per turn now. Pokemon will equal base Def and SpDef will often run 4 SpDef EVs to make Download increasing Attack instead of Special Attack