If you use the Free Shipping module this will make free shipping for all products.
If you want to offer free shipping on a few selected products only then you can do this by using the weight based shipping option.
Add a weight too all the products in the store
For free shipping make the items weight 0
For paid shipping make the items weight 10
Details of how to add weight options to a product can be viewed at:
You then need to enable the Weight Based shipping module and modify the settings so all items that weight 0 have free shipping.
We want to set up:
weight 0 = free shipping
weight 10 = $5
weight 20 = $10
weight 30 = $15
weight 40+ = $20
The code to enter in would be
0:0.00, 10:5.00, 20:10.00, 30:15.00, 999999:20.00
So the code to enter is
0:0.00, 10:(your price here for one unit), 20:(your price here for two units), 30:(your price here for thee units), 9999999:(your price for 4+ units)
Details of how to enable / use this module can be viewed at: