how do I enforce referential integrity between a many to many table and its parent tables - sql-server-express

Question

I have 4 tables.
Table 1 stores data for Airlines
Table 2 stores data for Destinations
Table 3 stores unique Routes(each route is a unique combination of airline and destination).
Table 4 records the prices of all flights on the routes in table 3. This is a many to many table as there are many flights by each airline to many destinations.
Table 1 = Airlines
Airline_ICAO_Code varchar(3) Not NULL Primary key,
Airline varchar(22) NULL
Table 2 = Destinations
Airport_ICAO_Code varchar(4) Not NULL Primary key,
Destination varchar(30) NULL
Table 3 = Airlines2Destinations
ID int IDENTITY(1,1) NOT NULL,
Airport_ICAO_Code varchar(4) Not NULL Foreign Key References Destinations(Airport_ICAO_Code),
Destination varchar(30) NULL,
Airline_ICAO_Code varchar(3) Not NULL Foreign Key References Airlines(Airline_ICAO_Code),
Airline varchar(22) NULL
Table 4 = Airlines2DestinationsPrices
ID int IDENTITY(1,1) NOT NULL,
Airport_ICAO_Code varchar(4) Not NULL Foreign Key References Destinations(Airport_ICAO_Code),
Destination varchar(30) NULL,
Airline varchar(22) NULL,
Airline_ICAO_Code varchar(3) Not NULL Foreign Key References Airlines(Airline_ICAO_Code),
Departure smalldatetime,
Price smallmoney
My problem is how do I enforce referential integrity between tables 3 and 4. This is necessary as the routes entered in table 4 must be present in table 3. I need advice as to what way to set up the Primary keys on Table 3 and Table 4 that will enable me to do this.
I am considering using a composite key on table 3 like this:
Primary Key(Airport_ICAO_Code,Airline_ICAO_Code)
or concatenating the Airport_ICAO_Code and Airport_ICAO_Code columns into a new column and dropping the ID columns.
If I use the Identity column as the primary key for both tables there is no guarantee that the ID for a route in Table 4 will match the ID for a route in Table 3.
Because of all these options I'm unsure of the best way forward. If someone could take the time to help out on this it would really be appreciated.
Thanks for any help offered
Edit:
After receiving advice from Marc in the post below I set up Table 4 as follows. However there is no primary key on this table. Would the ID column be better for the Primary key or could I use a Composite Key like this Primary Key (Airlines2DestinationsID,Departure).
Table 4 = Airlines2DestinationsPrices
ID int IDENTITY(1,1) NOT NULL,
Airlines2DestinationsID INT NOT NULL
FOREIGN KEY REFERENCES dbo.Airlines2Destiations(ID),
Departure smalldatetime,
Price smallmoney

Answer 1

Basically, if any one entry in table 4 always belongs to a single entry in table 3, I would just store the Airlines2Destiations.ID as a foreign key into table 4.
That way, each entry of table 4 is always clearly and uniquely connected to a single entry in table 3. Also: drop all the redudancy from table 4 - you only need the reference to table 3 - you don't need to repeat airline or airport codes and names.
Table 3 = Airlines2Destinations
ID int IDENTITY(1,1) NOT NULL,
Airport_ICAO_Code CHAR(4) NOT NULL
Foreign Key References Destinations(Airport_ICAO_Code),
Destination VARCHAR(30) NULL,
Airline_ICAO_Code CHAR(3) NOT NULL
Foreign Key References Airlines(Airline_ICAO_Code),
Airline VARCHAR(22) NULL
Table 4 = Airlines2DestinationsPrices
ID int IDENTITY(1,1) NOT NULL,
Airlines2DestinationsID INT NOT NULL
FOREIGN KEY REFERENCES dbo.Airlines2Destiations(ID),
Departure smalldatetime,
Price smallmoney