What is the sum of maxterms \$M4+M7\$?
For a Boolean function of four variables, the maxterm M4 equals 0100 which represents a + b' + c + d. Convention says to use 1 for complemented variables. The 0100 in binary notation equals 4 in decimal.

Similarly, the minterm m4 equals 0100, and convention says it represents a' b c' d'.

To compute M4 + M7, consider expanding the maxterms to their Boolean equivalents and performing the summation.

See also:


Karnaugh maps (Wikipedia)
Canonical form (Wikipedia)