How many alkanes having a molecular weight of hundred, are chiral? I successfully found 2, but the answer key says that there are 4 such compounds possible. I found:

3-methyl hexane
2,3-dimethyl pentane

We can't have a cyclic ring as the masses won't match. What else could be made folks?
You are essentially correct.  Of the nine isomers of heptane, only two are chiral.  However, if you count each enantiomer as a separate molecule, then there are "four" chiral isomers:


(R)-3-methylhexane
(S)-3-methylhexane
(R)-2,3-dimethylpentane
(S)-2,3-dimethylpentane