Why does Obi-wan let Darth Vader kill him at the end of Star Wars: Episode IV?

i think he tells luke later that he becomes more powerful in death
than he was in life.

Maybe Obi-Wan is already dead.

At the end of Episode III Yoda tells Obi-wan that his old Master has
mastered the force beyond the grave. (Quigon sp)  Yoda says he wants
to teach him how to contact him.  So in Episode IV I think ObiWan
knows he cannot directly beat Darth Vader.  He wants to give Luke time
to get Lei (Luke's sister, which Obi-Wan knows is his siter and Luke
does not).  I think he knows that he can communicate with Luke (Yoda
and Quigon) and is willing to let go.  Part of that Jedi training, let
go, not to hold on.

I may not have expressed the idea that well, but I think you can see
there is a connection from one episode to another.

I agree with ScubaJim, and I think that bit of information from Yoda
at the end of Sith is pivotal.  Up through the first three prequels
you never see any dead Jedi communing with the living but starting
with A New Hope you do.  Obi Wan knew he was going to have to guide
Luke as a spirit in much of what lay before him ("Trust your
feelings...").

As far as letting Vader kill him instead of fighting galliantly if
vainly, I think the information from Episode III also explains that
nicely.  Knowing that this communicating from the grave thing was a
new trick, the meditative passive posture Obi Wan takes on prior to
being cut down suggests he was getting his mind focused on his task,
getting in touch with the metachlorians (sp?), etc.

At least that's my current take on it.

The actor playing Obi-wan wanted to get out of the role.  In Star
Wars, he couldn't just move out of town.

I don't think the actor wanted to get out of the role.  He still had
to do the Ghost scenes.  He did get out of it by dying of liver cancer
I believe.

Because several stormtroopers had spotted him, and he couldn't fight
both them and Darth Vader at the same time. So instead, he prepared
himself to join the force, and allowed Vader to kill him.

Obi-Wan tells Vader that if he cuts him down, he will become more
powerful than Vader can possibly imagine. Obi-Wan, having been
communicating spiritually with his former Master Qui-Gon for some
20-30 years on Tattoine, would know that if he is separated from his
body, his spirit will have free range. He could then easily support
the good side of the Force, and Luke, from anywhere, no longer bound
by a body.