Just be honest and explain that it took that 4 months to find the role that fitted in with your career plans. Getting registered does not rely on getting a position immediately on finishing your degree but they are probably just trying to make sure there are no unexplained gaps due to omitting a role that would not fit in with the CEng ethics (running drugs or working as a paid assassin would probably be a couple of extreme examples).

As they didn't ask at the same time for further evidence I suspect that on receipt of a satisfactory answer the process will move to the next step but I can't say how quickly that will be. If they advise you that they are going to arrange an interview it may be quick to pass on that information but if they wait until they have a date for the interview it could be a significant delay (e.g, months rather than weeks).

Alasdair

There ought not to have any unexplained gaps in your work experience. This is one of the intial checks completed prior to your application being sent for review against UK-SPEC. A holistic check of the application is completed to ensure there are no ambiguity, omissions or gaps.

If you were unemployed after graduation for a period of time; state the facts in a concise manner. Even if you did volunteer work, express what learnings or experience you gained briefly. Valuable soft skills development can be attained from non-STEM roles; namely, document review, analysis, coordinating, planning, scheduling, coaching, mentoring etc. From the aforementioned, UK-SPEC competencies C, D or E may be credited if appropriately justified. 

Thank you Alasdair. I have received an update from IET to tell me that my explanation has been included in the application and the application has been passed on to the assessment panel. I don't know how much time it takes for a PRI call from this point onwards. 

Thank you for your guidance Ryan.