Is it obligatory upon Muslims to follow the 4 mujtahid imams (imam Hanifa, Shafi'e, Malik and Ahmad)?
If one follows the Hanafi way praying, can he then follow any other madhab in other areas of worship (eg. Hajj, fast)?
Can Muslims choose not to follow these imams?
My answers to your questions follow:
It is not a religious requirement upon a Muslim to follow the interpretations and explanations of any one or the other of the so-called 'four Imams'. The 'four Imams' were great scholars of Islamic disciplines and over centuries have attained the level of authorities in the interpretation and explanation of Islam. Nevertheless, following their interpretations and explanations of the directives of the Qur'an and the Sunnah is not a religious requirement. However, it is quite natural for an individual to consider the opinions of any one or the other of these Imams to be more reliable and, consequently, may choose to follow his opinions, on his own personal accord;
Yes. If a person feels that the Hanafi explanation of Sala'h is more accurate than others, while in Hajj the explanation and interpretation of another school is more accurate than he not only 'may', but should choose to follow the two different schools in the two respective issues;
Yes. If, in any given case, a person feels that the opinions of none of the four schools is accurate, then he would not follow the opinion of any of these schools.
I hope this helps.
December 18, 2001
 For more details on the issue of ascribing to a particular Muslim juristic school, please refer to a couple of my earlier responses titled: "Ascribing to a Particular Juristic School" (http://www.understanding-islam.com/related/questions.jsp?point=3&id=401) and "Fully Adhering to a Particular Juristic School" (http://www.understanding-islam.com/related/questions.jsp?point=3&id=404).
Answer published by Moiz Amjad