I am new for this group. We are working to covert a 25hp diesel tractor to electric. Our plan is to use 4 hub motors to make it 35hp E-tractor. Do you think this is doable?
How much torque do you need at what speed for how long? (not just power, watts, but actual torque needs--meaning, how hard does it have to push against the ground to do the jobs you need it to, worst case?)
If you need high torque at very low speeds for long periods, and also high speed capability, you might need two different sets of motors, one wound for high torque at low speeds, and one that is wound for high speeds, and a manual switch or system to autoswitch between them depending on the usage at the time. Each set would have to be capable of the full load of the system under those conditions. There are places like QSMotor that can probably wind any of their motors however you need them to, for the right price.
If you use a high speed motor for low speed high torque it will have a lot of waste heat, and may overheat.
To see how these things work you can play with the simulator at ebikes.ca; it's meant for ebikes but the way things work there apply to any system (and you can use the custom fields to simulate any system up to 2WD, to simulate 4WD you'll need to combine the two rear system needs into one, and the front into one, and use them as 2WD).
Then you will need a controller for each motor, and something to ensure they all operate the same way at the same time (some controllers can interlink for this, most can't).
Then you will need a battery and/or generator system that can supply the worst-case power continuously for the entire length of time the tractor needs it.
Not impossible, but almost certainly very expensive, and will almost certainly weigh a lot more than the original parts that did the same job.
This doesn't include whatever you'll need to do for the rest of the hydraulic systems--if they are run from the original engine, you have to keep that engine or replace it with one still powerful enough to run all those systems. Or you have to replace all the hydraulics with electrics, which is likely to be complicated and expensive. Not impossible, but....