What is meant by best four for DU?

The rules for calculating the Best Four vary depending on the course you want to take. Each course will have its own set of rules for computing it. Some courses will make some subjects mandatory, which means that whether your score is high or low in them, you must include them in those four subjects in order to be admitted to that course.

The four subject criteria will not be present in every course. Some courses, such as Physics, Chemistry, Electronics, Instrumentation, Polymer Science, and others, will only consider your PCM percentage, which is your marks in (Physics + Chemistry + Mathematics) divided by three. Some Biology courses will only consider the PCB percentage, which is (Physics + Chemistry + Biology)/3. 

Many things will be said to you about admission to the University of Delhi, but many of them will be incorrect. To obtain accurate information, one must visit the college itself. Attempt to visit various colleges on a regular basis. Remember to take chances even if your score is less than 90%. You will have a better chance of admission if you visit colleges on a regular basis.