Nice to see a Mersey Ferry in the top 3! For what it's worth, she shows up on AIS with IMO number 4900868.

Frans: Although useful, Grosstonnage is not totally reliable on matters like this.

IMO numbers rules are perfectly clear: no ships of war & troopships.
Here is an excellent explanation by Phil English:
The IMO number was introduced by the International Maritime Organisation as an aid to enhancing safety and as a measure to combat maritime fraud. The IMO has given LR Fairplay (now IHS Fairplay) sole authority for issuing IMO numbers which are based on the old Lloyds Register (LR) unique numbering system. The IMO number applies only to merchant vessels, hence there is no requirement for military vessels to be issued with them. However, LR and LR Fairplay did go through a phase of issuing LR numbers to military ships but, although LR numbers follow exactly the same pattern as IMO numbers, they are not strictly 'IMO numbers'
Brgds

Personally I found is strange to found an IMo number starts with 4, but info from GrossTonnage.com.

Hi Bob, not only ex-military essels have IMO numbers starting with a 4 but also active ones such as the minehunter HMS ATHERSTONE M38 which according to Marinetraffic.com her IMO. no is 4906666 which quite recently was in Malta, photos of her I posted her on this site,

There ARE a few vessels (ex-military eg http://www.shipspotting.com/gallery/photo.php?lid=1531410) whose IMO numbers start with a 4 - but this inland craft is not one of them.

ROYAL DAFFODIL is an inland navigation vessel without IMO number. Furthermore there are no IMO numbers starting with
number 4.

Imo: 4900868