The game only sees 2 engines, but the speed, fuel usage and battery recharge time is based on 4. It is only an issue for damage.
If you want to make your diesels more resistant to damage or even indestructible, it is fairly easy. Just open your Data/Zones.cfg file (notepad will do), find the entries [Diesel engine 1] and [Diesel engine 2] and bump up the hitpoint factor. Stock has I believe 100, TMO has it at 600, but you can easily bump it up to 2000 or more each.
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