How much SHOULD I feed my 4 year old miniature dachshund?

A good quality kibble will tell you on the back what to feed an adult dog by their weight, but I would guess 1/2 to 3/4 cup dry mixed with about 2-3 ounces of wet (canned, etc) a day.
My dogs have always had all the kibble they want, on the theory that they won't wolf it down out of fear of not getting enough, and it has always worked, even with food inhalers like retrievers. I tend to mix the food so that extra kibble is near the bottom and they leave some.
A lot depends on how much exercise they get.
Also, cheap dog food just has a lot of filler that comes out the other end, really. You have to lug more home, feed more, so it isn't even a savings. The better the food, the smaller the poops, and more nutrients they get too.

You should talk to your vet for proper instruction.

The recommendations on the back of the bag are just that, a recommendation. Lots of dogs get fat off the info on the back of a bag. The bag also doesn't include amount of exercise the dog gets each day, which is something that should be considered!