4.Read the following material before answering the questions that follow:  In general, the multiplication of $n$ identical numbers $a$, denoted $a^n$, such as $2^3=8$, when, 3 is called the logarithm with base 8 of 2, denoted ${\log }_{2}8$ (i.e., $3={\log }_{2}8$). In general, if $a^n=b(a>0,a\ne 1,b>0)$ then $n$ is called the logarithm of $b$ with $a$ as base, denoted ${\log }_{a}b$ (i.e., ${\log }_{a}b=n$). If $3^4=81$, 4 is called the logarithm of base 81 with 3, denoted ${\log }_{3}81=4$. (1) Calculate the value of each of the following logarithms: ${\log }_{2}4$ = _______; ${\log }_{2}16$ = _______; ${\log }_{2}64$ = _______. (2) Observe what relationship is satisfied between the three numbers 4, 16, and 64 in (1)? What relationship is satisfied between ${\log }_{2}4,{\log }_{2}16,{\log }_{2}64$? (3) Based on (2), can you write a general conclusion? ${\log }_{a}M+{\log }_{a}N=$______________($a>0, \mathrm{and} a\ne 1, M>0, N>0$)