To create the SQLite database using SQLAlchemy with the specified tables and requirements, follow the steps below.

First, install SQLAlchemy if you haven't already:
```bash
pip install sqlalchemy
```

Then, you can use the following Python code snippet to create the SQLite database with customers, orders, items, and product tables based on your requirements:

```python
from sqlalchemy import create_engine, Table, Column, Integer, String, ForeignKey, Numeric, MetaData
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

# Create the SQLite database
engine = create_engine('sqlite:///system/genai/temp/create_db_models.sqlite')
Base = declarative_base()

# Define the tables
class Customer(Base):
    __tablename__ = 'customers'

    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String, nullable=True)
    balance = Column(Numeric, nullable=True)
    credit_limit = Column(Numeric, nullable=True)

class Product(Base):
    __tablename__ = 'products'

    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String, nullable=True)
    unit_price = Column(Numeric, nullable=True)

class Order(Base):
    __tablename__ = 'orders'

    id = Column(Integer, primary_key=True, autoincrement=True)
    customer_id = Column(Integer, ForeignKey('customers.id'))
    notes = Column(String, nullable=True)
    customer = relationship("Customer")
    amount_total = Column(Numeric, nullable=True)

class Item(Base):
    __tablename__ = 'items'

    id = Column(Integer, primary_key=True, autoincrement=True)
    order_id = Column(Integer, ForeignKey('orders.id'))
    product_id = Column(Integer, ForeignKey('products.id'))
    quantity = Column(Integer, nullable=True)
    amount = Column(Numeric, nullable=True)

# Create the tables in the database
Base.metadata.create_all(engine)

# Create a session to interact with the database
from sqlalchemy.orm import sessionmaker

Session = sessionmaker(bind=engine)
session = Session()

# Insert some sample data
customer1 = Customer(name='John Doe', balance=1000, credit_limit=1500)
customer2 = Customer(name='Jane Smith', balance=2000, credit_limit=2500)
product1 = Product(name='Product A', unit_price=10.5)
product2 = Product(name='Product B', unit_price=20.75)

# Add the objects to the session
session.add_all([customer1, customer2, product1, product2])
session.commit()
```

You can run the above code to create the SQLite database with the specified tables and insert some sample data into the customers and product tables. Feel free to adjust the code according to your specific requirements or data model.