A 1/600 scale 4000 ton cargo ship will probably be 6-7" long. Heroics and Ros now carries the old Skytrex 1/600 coastal forces range, which includes merchant ships for the MTBs and MGBs to hunt or defend. I found this TMP page with some pictures of Bezmozgu7's nicely painted examples of these. The newer company PT Dockyard also stocks coastal forces which should include targets like tramp steamers.
In 1/1200 scale I only know of the old Hallmark line (sold by Magister Militum and others), and all the expensive old gems like Navis, Neptune, Superior, etc. (sold by aggregators like this). You might be able to find something on Shapeways.
There are a lot of choices in 1/2400:
C-in-C sells some merchant ships in the US and British WWII ranges. The most appropriate model is probably the MF-23 Tramp Steamer, which you can see in photos on this TMP page.
GHQ also sells 1/2400 non-combatants and semi-combatants in their WWII Micronauts range. Start with the UK category, click around and see if one looks right.
Viking Forge may have some suitable ships, but not all of them are photographed, so it may be hard to tell. The most likely place to find merchant-y ships is in the US WWII range.
Panzerschiffe sells "merchant ship groups", one of which might contain a suitable miniature. I bought a couple of these groups to use as targets, objectives, scenery, etc. in naval games, and found they were *very* plain and needed quite a bit of dressing up to look nice. I added wire cranes, boats, anchors and anchor chains, and painted on portholes. No matter what I do I still don't like them as much as the C-in-C and GHQ civilian ships I have.
- Ix
rebases some old soldiers.
