Service Record Database Design Approach

I'm trying to build a table structure for this database. Heres some background information about the business:

The business is a service business. We visit the customer's location and run tests on whatever water systems they have. Each customer is unique in that they could have any combination of systems at their site. They can also have more than one of the same type of system. The test results are the data that I need to record and store for future access. Each customer is visited on average once or twice a once a month. So there should not be more than 1-2 entries of data for each system for each customer per month.

For example
customer RUTGERS might have two systems labeled HWS and CHWS.
customer BMSQUIB might have three systems labeled HWS1, HWS2, and CHWS.

What I need to do with this information is go into the records of service visits, and retrieve for example, the last 4 visits of a specific system and prepare that information to be printed in a report along with their contact information.

I have come up with three tables to do this:

Customers Table contains:
Customer ID (pk), contact information data.

Systems Table contains:
System ID (pk), Customer ID, System Name

Service Records Table contains:
Record ID(pk), Date, System ID, Data

My thoughts were to have a table to contain customer information(each customer with a unique ID), a table to contain system information for each customer (each system has a unique ID), and a table to store the results of every service visit for each system(each individual visit has a unique ID)

Please critique this table design. If you think its sufficient, perhaps you could lead me in a direction pertaining to how to retrieve data on the most recent 4 visits (last 4 entries) for a specific SINGLE system from the Service Records table. I would assume that you would need to use a query and then get the data from that and put it into a form.