"In 1941, FDR said he looked forward to a world founded upon four essential freedoms: freedom of speech, freedom of worship, freedom from want, and freedom from fear." (B. Obama - The Audacity of Hope)
1) What does the expression freedom from want mean?
2) Is there a rule in English that after prepositions verbs must always come in gerund form? If the answer to this question is affirmative, I guess want here cannot be considered as a verb, rather a noun.
3) Maybe here the meaning of want is "personal defect" or a "defect of character" (from - Definition from the Merriam-Webster Online Dictionary
4) If one changes freedom from want to freedom of want what is the new meaning?
1 &3. In this case, the word "want" is definitely a noun, and it means this: : grave and extreme poverty that deprives one of the necessities of life (source: Dictionary and Thesaurus - Merriam-Webster Online)
In other words, people should not suffer from this type of poverty.
2. Yes, after a preposition, you need an object of that preposition, which is a noun. Remember that a gerund is the -ing form of the verb that acts like a noun.
4. That doesn't work. With "freedom of want" it sounds like you have the freedom to want things, and that's not at all the same thing as being from from this feeling of terrible poverty.