"使自乘二次" can be translated into English as "to raise it to the second power (or square it) twice" or more precisely in a mathematical context, "to square it and then square the result again" (though the latter is a bit more verbose).
However, if we're aiming for a concise and common mathematical expression, "to raise it to the fourth power" would also be correct in a sense that squaring a number twice (i.e., multiplying it by itself twice, then doing it again to the result) is equivalent to raising it to the fourth power (since $ (x^2)^2 = x^4 $).
But if strictly adhering to the literal meaning of "自乘二次" (which implies squaring once and then squaring the squared result again, though mathematically equivalent to raising to the fourth power), the most straightforward translation capturing the process would be:
"to square it and then square the squared result"
For everyday or less technical contexts, "to raise it to the fourth power" would be perfectly acceptable and more concise.