I used Smogon's Damage Calculator, and any stat below 50 with a decreasing nature will be 4 if level one (except Shedninja; it will always be lvl 1). There are far too many to list, so go to http://play.pokemonshowdown.com/teambuilder, press New Team, Add Pokemon, and then press the stat you want to see the lowest. Then scroll to the bottom, then scroll up until you wind a Pokemon that has the base stat greater than 49. Every Pokemon below the Pokemon will have the lowest possible stat of 4 (excluding Shedninja) at level one with a decreasing nature. Sorry if it is confusing, I will try to clear it up later.
EDIT: I just forgot, the lowest stat for Speed is 2 if they are holding an Iron Ball or any EV-increasing items.
Tl;Dr Any Pokemon with a base stat lower than 50 and has a receding nature will have the lowest base stat of 4 at level 1 (except Shedninja, which will have 1 HP).