There was a major update (an update actually, as MT4 is end-of-life for quite a few years and does not receive updates) for MT4 to be compatible with newer versions of MetaEditor, which is the same for 4 and 5. That came with changes in IDE, probably relevant to order types. MT4 does not support the new ones, but the enumerations and macros should be the same so if the script does not use the actual enumerations but something like if(OrderType<2) it can easily get in trouble.