I would go with 4 HP / 252 Def / 252 SpD. Chansey’s HP stat is way higher than its SpD stat (and any of its other stats), and since EVs add a common value to the stat instead of multiplying you benefit more from investing in a small stat, so 252 EVs in HP aren’t much of a difference.
Because of Chansey’s design, most of the time it won’t take any physical attacks anyway if you’re playing right. And you can always use a Bold Nature for extra support against surprise physical attacks.
Even though Smogon recommends the latter EV spread, top Uber players (1630+ Elo) tend to use the former spread most of the time (Source).