I guess this question is more general than this meta specific but when doing the EV spread, is it better to put 4 EVs into HP instead of Def/SpDef? Is there any difference? And this is directed more for when I want to give defensive spreads and can't decide whether it's better to put more EVs into HP or the defenses.
There is no overall best place to put those last 4 EVs, it depends entirely on the Pokémon. Putting EVs into HP rather than Defense or Special Defense gives you more bulk overall, whereas placing them into separate defences gives you more bulk in the defense stat you place them in.
However, there are many other factors to consider. Putting EVs into the lower defense stat gives you a higher percentage increase than if you put those EVs into the higher defense stat. For example, maximising Blissey's Special Defense gives her only a 20% increase in special bulk, but maximising her Defense stat gives her a 95% increase in physical bulk, a much larger difference and clearly more beneficial overall.
There's also the factor of how your HP is divided. If your HP is divisible by 2, 4, 8, 16, or 32 (depending on how weak you are to Stealth Rock), then you'll be KO'd by Stealth Rock if you switch in that many times. If your HP is divisible by 8, you'll be KO'd by Leech Seed, Burn, or Poison in 8 turns. If your HP is divisible by 10 and you're using a Life Orb, you'll be KO'd after attacking 10 times. If your HP is divisible by 16, you'll be KO'd by Sandstorm or Hail in 16 turns. And if your HP is divisible by 6, you'll be KO'd by 2 layers of Spikes if you switch in 6 times (all these are assuming you don't lose HP through any other means). Whereas, if your HP isn't divisible by any of these numbers, you'll survive with a few extra HP, and ultimately live another turn. Therefore it's usually best to avoid being divisible by these numbers, but keep in mind you do gain extra HP via Leftovers if you HP is divisible by 16, etc. (You can read more about this
here).
Finally, if your Base stats for each defense is equal, then you may want to put an extra EV point in Special Defense to give Genesect a Physical attack boost instead of a Special one (this is only a good idea if that particular Pokémon / your entire team prefers it if Genesect gets a Physical attack boost).
And, for future reference, if you have a competitive question that isn't specific to one tier such as this one, ask it
here. I hope this helped :)