about DLL and mt4 mt5

I finished a dll, it can run under MT4, but cannot be run under MT5, what diff between mt4 and mt5 for dll? so strange
codeidea:
I finished a dll, it can run under MT4, but cannot be run under MT5, what diff between mt4 and mt5 for dll? so strange

strings: ANSI(MT4) - unicode(MT5)

datetime: 4 bytes integer(MT4) - 8 bytes integer(MT5)

can I write a dll which can be used under MT4 and MT5 in same time?
codeidea:
can I write a dll which can be used under MT4 and MT5 in same time?
Yes. With different entries