Unfortunately if you can't run macros, and you can't change the security settings to allow you to run macros (Tools -> Macro -> Security), your only option to quickly change A4:Y270 to 0's is this:

Upon opening your file, look to see if G1 > G2. If so, then press F5, and in the reference box type A4:Y270 and click OK. Type the number 0, and press CTRL+ENTER. That will put a 0 in every cell selected.