You do a device check in the appdelagate and this is where you can specify to run specific things like higher res graphics or different code based on the platform.
It's not that hard to port an app to the iPad... if you use points and not pixels in your application code. There are several methods to use high res graphics for the iPhone 4 and the iPad. Sounds like you have the right approach if you have no budget for the iPad version.
For me the time to develop the iPad version of my game was six months. The iPhone version was 1 week. The code base is the same you just have to tweek some things like Retina displays and some graphics. But the majority of the code will not change.
Are you sure you don't want to make it for both? It's not that hard or costly! I would develop the iPad version first then scale down for the iPhone... It's easier to "Design Big" then scale down then it is to scale up... especially graphics! Just saying...