That's one line of code, not one instruction (by my count, it's like 10-12 instructions - load, sub, rightshift, add, store - times 2 because on an 8-bit avr, and 16-bit operands...)
As an aside, dividing by 3 is much worse, while dividing by 4 (or other powers of 2) is not - division on AVRs is implemented in software and slow.