Walmart's $4.00 price is for the drugs offered in this program. You do not have to be low income to benefit from the program. According to Walmart's statement:
Wal-Mart's Commitment to Lower Health-Care Costs
Generic drugs use the same active ingredients and are as safe and reliable as their brand-name equivalents. Many of the drugs available in the $4 prescription list are generics. In addition to those on the list, the pharmacy at Wal-Mart carries a range of generics that offer significant cost savings over their name-brand equivalents. Learn more about the benefits of generic drugs.
Medicare coverage offers significant savings to many Americans who are eligible. Learn more about Medicare, eligibility rules, and programs available to save you money on your health and prescription needs.
* $4 prescriptions are for up to a 30-day supply of a covered drug at a commonly prescribed dosage for $4 per prescription fill or refill. Your participation in certain prescription drug coverage plans may entitle you to pay even less than $4 for certain prescriptions. If you are eligible, you will be charged the lowest applicable amount. Certain drugs are priced higher in CA, CO, HI, MN, MT, PA, TN, WI, and WY due to state laws. Program not available in North Dakota. You can get these prescription drug savings whether or not you have any prescription drug coverage through your company, under Medicare or any other plan. The list of covered drugs is subject to change. Not all prescription drugs are covered by this program. Only prescriptions initially filled in person at a participating pharmacy are eligible for the $4 rate; refills must also be picked up in-store, but may be ordered in person, online or by phone. This program is not available for prescriptions filled by mail order. See your Wal-Mart pharmacist for more information.
I believe that Target's, Myer's and other similar stores follow the same policy in regards to their list of $4.00 medications. You can actually get a list of the medications they offer under their program on any Walmart online site as well.