With infrastructure now being on the provincial level, the USSR will be able to attack along the entire front as those impassable provinces won't exist. The only possible constraint will be supply bottlenecks limiting the number of divisions that can operate north of the Mannerheim Line front.
Hopefully this will be limited to historical few divisions, though.
HoI 3 solution would have worked near-perfectly otherwise, but the AI couldn't be lured to attack at the Leningrad-Viipuri front against too difficult odds since it was not programmed to fight with attrition tactics. All in all Winter War cannot be really simulated well in a general grand strategy game like HoI IV since the Finns operated with battalion-level units against Soviet regimental and even divisional level forces on regular basis.
Replicating situations like the battles of northern shore of Laatokka where Finnish IV Corps, essentially two worn-out infantry divisions, first stopped and encircled two Soviet divisions while simultaneously holding off 11 Soviet divisions that were attacking along the region.
http://www.talvisota.fi/files/kartat/wg1079-01.jpg
I'm highly sceptical about the chances to mimic results like this in HoI IV. And it might not even be necessary. On a grand scheme of things getting the Winter War "right" only requires proper policy and decision threes where the war affects the general course of WW2 in Scandinavia and in the Baltic region, and allows the Soviet Union a chance to experience the disastrous effects that Stalin's officer purges had to the RKKA first-hand before the Nazis invade the USSR. If the game gives the Finns a chance to wiggle their way through the war without becoming part of the Soviet Union, good, but generally speaking only the diplomatic front and the army reform programs that Soviets historically initiated after the Winter War are really important for the big picture.
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