Atleast during the streams, if the german player bought stuff from the soviet union, romania and/or italy it didnt require convoys according to the trade window. That might of course be an display bug, but as far as i know there was landtrade in HoI III aswell. I can imagine tho, that you have to have an direct connection to their country for that of course.
There was overland trade in HOI3, and the lack of convoys in those windows is not a display bug, but an indication that the same rules apply. If your capital is on the same continent as the seller's capital, then the resources you trade come to you overland without convoys. So Germany can trade with Soviet Union, Romania and Italy overland.
This always produced some odd results. Germany could trade with Portugal overland, and this still applied even when there was a war with France and the only possible way to travel would have been through a hostile country and pass through the frontlines.
UK and Japan could trade with other countries overland, even without land connections to the rest of their respective continents. And Japan could trade with Tibet and have it's resources pass through China while at war.
Despite these oddities, the rules work quite well to allow the Axis countries in Europe to trade amongst themselves, without sending convoys through the English Channel and/or getting blocked at Gibraltar.
There are no concerns over the length of the route overland, where it might be quicker by sea - distance is not taken into account in the overland trade. The resources basically teleport from one capital to another. I guess it's abstracted that they travel by train, river barge and maybe even by coastal ships to get where they need to.
Another slightly odd result is that before war with the Allies, Japan can't get Rubber from Malaya nor oil from DEI. The colonies always ship their resources by convoy back to London and Amsterdam. Then Japan has to send convoys to get them from Europe. But it wasn't a biggie, given that this only really happened at peace when those convoys wouldn't be sunk.
There are two small changes which affect how this all works in HOI4. Firstly, I'm pretty sure there was simply an Asia continent, and Japan could trade with Iran and Iraq overland. Now there is a Middle East continent shown on the trade and diplomatic screens. That would require Japan to use convoys, so if during the war Iran or Iraq go fascist, Japan gets no easy oil. It can trade with them, but would have to send convoys that would be attacked.
Secondly, India is no longer just a colony, so the requirements to ship all resources back to London no longer applies. British Raj can trade directly with other countries. And specifically this means China can trade overland with British Raj, so there is a sort of Burma Road that Japan cannot blockade. But it's not OP because China still can't get oil from the Middle East, and therefore will always struggle to mechanise it's army or build a decent air force.
It's a couple of small, but important, changes that make the war over resources in Asia better.
