1. Amperage does not directly relate to speaker power handling. Amplifiers are rated in terms of power output in watts, not amps. The amperage rating of an amplifier refers to the maximum amount of current it can safely deliver to a load, not the amount of power it can produce.
2. Impedance mismatch. The impedance of a speaker is a measure of its resistance to electrical current. An 8 ohm speaker is designed to work with an amplifier that has an output impedance of 8 ohms. Using a speaker with an impedance that is different from the amplifier's output impedance can result in reduced power output, distortion, or damage to the amplifier or speaker.
3. Power handling. The power handling of a speaker refers to the maximum amount of power it can safely dissipate without being damaged. A 400 watt speaker is designed to handle up to 400 watts of power. However, the power output of an amplifier can vary depending on the impedance of the load it is connected to. Using a speaker with a lower power handling rating than the amplifier's output power could result in the speaker being damaged.
In short, using an 8 ohm 400 watt speaker on 4 350 amp outputs is not recommended and could result in reduced performance, distortion, or damage to the amplifier or speaker.