As mentioned above, steering with the hydraulic system off (engine not running) takes more effort, but should still be effective. This would be true of both 4 and 5-port variations as long as the other hydro levers are not stuck open or in float.
The most common reason for steering to be absent after months of the driveshaft not being turned by a running engine is the fluid in the column might drain back down into the reservoir in the transmission, which is a low point in the system. There is also a check valve in the steering valve itself that makes that valve act as a pump to keep steering function when the tractor is not running. Here is what the manual says:
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This function is meant to allow for steering to continue after an engine power loss...it may not continue to work well after the fluids drain back after months of parking/inactivity. Some of my tractors would go 90 days or more without an issue with 'manual' steering, and other would not go that long. A freshly rebuilt steering column and valve should work as described above for weeks at least....
Any air in the hydraulic system will cause a loss of function of course.