Bdtyktl

I came up with this riddle a few years ago, and so far very few people have been able to solve it:

Think of a four-letter word (“_ _ _ _”) such that adding an A to the beginning (“a _ _ _ _”) changes the meaning but not the pronunciation. (It's a normal word that I guarantee anyone with at least a middle-school education will have heard.)

If you answer, please use spoiler tags.

I'll go with

isle - an island - which is pronounced the same as aisle - a walkway.

Perhaps another couple of solutions are

Iran, which when you add an "a" becomes airan, which, according to Webster's Third New International unabridged, is pronounced the same as one possible pronunciation of Iran, but means a Turkish drink made from fermented milk

And

Erie (the city) and aerie which can also be pronounced the same, but means a nest.

However, these are

capitalized words, so perhaps they don't quite fit. And though middle school students probably know Erie and Iran, and maybe even aerie, it is doubtful they know airan.

Probably not this, but worth writing down:

Aron and Aaron are names, referring to different people but pronounced the same.

I'd go with Tom's answer, but would like to mention another possibility, which is

Ide - a freshwater fish.

Which of course yields

Aide

As the other word. Neither is one that would necessarily be known by everyone though.

I believe this wasn't covered yet:

long (/lôNG,läNG/): distance/lasting.

along (/əˈlôNG,əˈläNG/): together/accompany.

and

rose (/rōz/): flower.

arose (/əˈrōz/): emerge.

Updated:

iris [aɪrɪs]: the round coloured part that surrounds the pupil of your eye.
airis [āīɾis]: oar, paddle (an instrument for rowing a boat).