in a famous Christmas film Santa is saved from a life in institutions
by four words. What are they? And where are they?

In the 1994 remake of "Miracle on 34th Street," a kindly old man
claims to be the real Santa Claus. A hearing is held in court to
determine whether or not to send him to an insane asylum.

The court case is won by Santa's supporters when it is shown that the
United States government, by placing the phrase 'In God We Trust' on
the dollar bill, confirms the existence of a higher authority.

"Dylan McDermott is a lawyer, and he agrees to defend 'Santa' at his
insanity hearing... The trial ends in a slightly different manner than
the original. Instead of the postal service delivering letters
addressed to Santa Claus to 'Santa' giving the judge a way to prove
his identity, as in the original, Dylan uses a dollar bill. On a
dollar bill it says 'In God We Trust.' If the federal government can
declare an unseen entity to be real enough to trust, then how could a
state judge say that unseen entities don?t exist? How could a judge
declare that Santa didn?t exist?"

Epinions: Miracle on 34th Street 
http://www.epinions.com/mvie-review-7795-41F8DDF-3A0712F7-prod4

So, the answer to your question:

The film: "Miracle on 34th Street" (1994 version)
The four words: "In God We Trust"
Where are they? On the United States' one-dollar bill.

Google search strategy:

Google Web Search: "miracle on 34th street" "dollar bill" "in god we trust"
://www.google.com/search?hl=en&q=%22miracle+on+34th+street%22+%22dollar+bill%22+%22in+god+we+trust%22

I hope this is precisely the information you need. If it is not, or if
anything is unclear, please request clarification; I'll be glad to
offer further assistance before you rate my answer.

Best regards,
pinkfreud

the answer couldn't be better then this i think. precisely the answer
you need for that question.

dear martinitoren,
you should rate this great answer so that pinkfreud can collect her
fee for the effort she put into it . . . .  just a head's up in case
you didn't catch that part . . .

: )

ts