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If a LiFePO4 needs to be jumped, then it's probably already toast. I've killed a 4.2 Ah battery just in the time it took to measure various electrical loads with the engine off; probably 15 minutes of testing. I would only recommend a 4.2 Ah battery for vehicles with very modest electrical requirements and parasitic drains, such as a motorcycle. 20 mA is a common parasitic drain (car off and parked), and that would give about 1 week of standby time and not leave enough charge left to start the vehicle.
I consider 20 Ah to be minimum for most vehicles. This would allow engine off coasting, listening to the radio with the car off, or parking the vehicle for several weeks.
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