If the Contractor was in delay by 4 weeks and the Employer then changed the Works Information during this delay (i.e. after the Contractors delay) resulting in a critical delay of an additional week, is the Contractor able to move the Completion Date out by 5 weeks?
I am afraid not (assuming you are talking as the Contractor). The Contractor delay has moved planned Completion by four weeks. The Employer delay then moves planned Completion by one week, and following the rules of clause 63.3 Completion Date would only move by one week.
If you did (or could have) mitigated the initial Contractor delay before the Employer delay came along then yes you would be entitled to a five week delay to planned Completion/Completion Date. This all assumes you also have a regular Accepted Programme for both Parties to call upon to assess any delays with.