Will a mk5 fiesta engine fit mk4 cheers?

No, a Mk5 Fiesta engine will not fit a Mk4 Fiesta (or a Mk4 Fiesta "Cheers"). The engine bays and mounting points are significantly different between the two generations. You would need extensive modifications, likely including custom engine mounts, potentially modifications to the chassis, and possibly other significant changes to make it work, and it's very unlikely to be a practical or worthwhile undertaking.