My current feeeding routine is every four days, with a substantially-sized clam. Although the octopus seems content, I was wondering if this was too infrequent; i.e. will it stunt its growth or development?

It will possibly slow down the octo's growing which may not stunt it but depending on temp may allow it to live a bit longer... what is the temp now?

i do think you could feed every 2nd day and vary the food to other tings too.. crabs work well :)

My impression of marine life is that you cannot leave it to starve while you go on holiday for 14 days (like it used to be recommended to do for freshwater fish, to avoid the neighbours sucumbing to over-feeding them!).

The only cephalopods I kept were cuttlefish, from eggs, and the important thing was to feed as much as possible to keep them feeding, otherwise they just faded away.
As I usually had several at one time there was another incentive to make sure they were satisfactorily fed each day!
Frequent feeding also helps to build your relationship with your pets?

The tank temperature is about 75 degrees farenheit. Would it be wise to reduce the temperature to prolong its lifespan? Also, would it be possible to feed even less frequently in an attempt to prolong its life?

You probably could lower the temperature to the lower seventies without too much trouble, and that would be desirable.

Most people feed their octopuses either every day or every other day. As Colin says, it is important to feed a variety of things to approximate what the octo eats in the wild - there's a wide choice -shrimp, crabs, mussels, clams, scallops, fish, etc. It seems a good idea to give your octopus some live food that it can hunt, too.

Nancy

Nancy's right.