|Ugh. Not him again.|
In a nutshell, folks generally seem to agree that the thief class's fast level advancement compensates for their sub-prime hit dice. I don't agree with that. Sit tight and I'll tell you why.
I'm of the school of thought that your hit point potential (aka hit dice) at the outset of your career--which is to say at level 1, not level 4--are a product of your background, and therefore there is some basic assumption about your background that justifies your hit points at inception. For instance, Fighters are hardened from combat training and perhaps even actual warfare--especially if we take the "veteran" level title literally,as some do--and thus warrant heightened hit dice. Magic Users, meanwhile, earn their pathetic pyramidal hit dice either from a sedentary life of study, or from contact with the soul-sucking arcane forces of the universe, or some other wussifying factor.
But what is there about the thief class that should make them universally sub-normal in the HP-category? Sure, some of' em--even a lot of them--might be underfed pipsqueaks who stole their last meal from a fruit cart. But even so, thieves are out-and-about, actively sneaking around, climbing stuff, and, if they come from the illicit branch of the class, they're probably even getting in more than their fair share of fights: administering beat-downs on deadbeats and narks when they're not rumbling with rivals; whatever it takes to get ahead in the world. They are survivors. All of which should at least allow for a normal hit point potential at the outset; but saddling thieves with d4 HD negates this sort of thief. While there is definitely room for malnourished street urchins, why should they be the sole model for the class?
Admittedly, my counter-argument assumes that a Normal Person gets 1d6 HP, a tenet of AD&D that might not have existed in Basic. Holmes, to the best of my knowledge, was mum on the topic; the only corollary I've found being the bandits in the monster section, though they get 1d8 hit points, making them actually better off than their AD&D compatriots--and making d4 thieves look even more pathetic by comparison. I can't speak to the Original rules or the Moldvanian or Mentzerian versions; they may very well insist that all normals get d4 hit dice. If such is the case, you may ignore everything I just said.