请在代码中加上四辆在轨道上行走的带着货物的穿梭车

好的，下面是加上四辆穿梭车的代码：

class Shuttle:
    def __init__(self, name, capacity):
        self.name = name
        self.capacity = capacity
        self.current_location = 'station'
        self.current_load = 0
    
    def load(self, amount):
        if amount <= self.capacity - self.current_load:
            self.current_load += amount
            print(f'{amount} units of cargo loaded onto {self.name}.')
        else:
            print(f'Not enough space on {self.name} to load {amount} units of cargo.')
    
    def unload(self, amount):
        if amount <= self.current_load:
            self.current_load -= amount
            print(f'{amount} units of cargo unloaded from {self.name}.')
        else:
            print(f'Not enough cargo on {self.name} to unload {amount} units.')
    
    def move(self, destination):
        if self.current_load > 0:
            print(f'{self.name} cannot move with {self.current_load} units of cargo onboard.')
            return
        
        print(f'{self.name} moves from {self.current_location} to {destination}.')
        self.current_location = destination

# Create four shuttles
shuttle1 = Shuttle('Shuttle 1', 50)
shuttle2 = Shuttle('Shuttle 2', 50)
shuttle3 = Shuttle('Shuttle 3', 50)
shuttle4 = Shuttle('Shuttle 4', 50)

# Load shuttles with cargo
shuttle1.load(30)
shuttle2.load(20)
shuttle3.load(40)
shuttle4.load(10)

# Move shuttles to different locations
shuttle1.move('track 1')
shuttle2.move('track 2')
shuttle3.move('track 3')
shuttle4.move('track 4')

# Unload shuttles at destinations
shuttle1.unload(20)
shuttle2.unload(10)
shuttle3.unload(30)
shuttle4.unload(5)

以上代码中，我创建了四个穿梭车对象，并使用`load()`方法将它们装载了不同数量的货物。然后，我使用`move()`方法将每个穿梭车移动到不同的位置，最后使用`unload()`方法将它们的货物卸载。