The term stereogenic center or stereocenter:

A stereogenic center is defined as an atom on which an interchange of any two atoms or groups result in a new stereoisomer, When the new stereoisomers is an enantiomer ,the stereocenter is called chiral center. All stereocenters are not tetrahedral.

First and second are enantiomers (non superimposible mirror images), Hence the steriocentre is a chiral centre.

III and IV are not enantiomers. they are diasteriomers hence in this case sterio centres are not chiral centres. Also these are not tetrahedral. Thus, all chiral centres are sterio centres but all steriocentres are not chiral centres. If a molecule contains only one chiral centres it must be chiral. Molecule containing two or more 

chiral centres may or may not be chiral. For example: meso tartaric acid has two chiral centres but it is achiral.

Realated Questions:

(1) How many cyclic isomers possible of (C6H12) Hexene?

(2) How many cyclic isomers possible of (C5H10) pentene?

(3) Write the major and minor product of hydration of 1-Methylcyclopent-1-ene in acidic medium?

(4) Why enol form of Beta Dicarbonyl compound more stable than those of monocarbonyl compounds?

(5) Why enol form of phenol is more stable than its keto isomers?

(6) What is Imine-Enamine tautomerism?

(7) The term Asymmetric center and chiral center :