This article is from the FAQ, by with numerous contributions by others.
For (1) above, although no job offer is necessary, it is advisable to
have evidence of pre-arranged employment commitments with an employer
in the USA or submit other evidence that the alien is seeking to enter
the USA to continue work in the area of extraordinary ability.
For (2), a job offer from a University or company is necessary. In
particular, the letter from the employer must state that the alien
has been offered a tenure-track faculty position or similar position at
a University, or a comparable position with a private company engaged in
research. The alien must be the beneficiary of a petition filed by the
employer with the INS. In addition, if the employer is a private company,
the employer must show that they have documented accomplishments in the
academic arena and that they employ at least 3 people engaged in full-time
research. Finally, the alien must demonstrate evidence through letters from
past/present employers that he/she has at least 3 years of full-time
experience in the field of outstanding ability. Research or teaching
performed while studying for an advanced degree may be used as experience
provided the degree was obtained and the alien had full responsibility
for the teaching or research and the teaching or research was found to be
outstanding by recognized experts in the field.
For (3), job offer is necessary.
For (4) again no job offer is needed. However, in some instances, if
the alien is in the USA, offer of employment may be needed to provide
evidence of financial support.