@chuet - that is an S3 body style, not an S4 body style. No, the unit number on the chassis plate and frame number would not be the same - they were individually numbered (at least during the Lotus run). If the chassis/unit number is later than 2649, it would likely be an S3 made by Caterham or a replica that got fitted with Lotus badges (not unusual). Someone else here may recognize some of the bits in the picture and be able to help - but the rear lights, dashboard, and position of the gas filler all suggest to me it was either not built by Lotus or was changed later.
To your S4 question, there were few than 700 built I think - so the Lotus numbering would end in the 3300s, I think.