About how much is a 1970 mustang worth?
There is a lot of information I still need to provide an accurate answer. For starters, what body style (coupe, fastback, convertible) is it? If it's a coupe, is it a Grande? If it's a fastback, is it a Mach 1? For $4,500--and being an automatic--I'd guarantee it ain't a Boss 302 or a Boss 429 (Boss 'Nines bring six-figure prices at Barrett-Jackson).
And, once you've worked out the body style, what engine does it have? Big-blocks add considerably to the value and the fuel consumption.
You'll also need to take options and color combination into consideration. Air-conditioning is very desirable, as are power steering and brakes. A red Mustang is a much easier sell than one that's Medium Lime Metallic.
And then the overall condition of the car comes into play. It might be a good deal at $4,500, but if it has significant rust or body damage, you might have to pay $4,500--or more--to fix it. Also look out for missing or incorrect parts, and do research as to how hard to find they are before you buy.
Of course, if you intend to "resto-mod" it, missing or incorrect parts might not be too big an issue.
Based on what little you gave me to go on, $4,500 seems reasonable, if not a screaming deal. Good luck.