My 4yr old w/adhd has a room full of toys that he never plays with. Is this common w/adhd

This is very common because he is lacking the ability to stay focused and therefore becomes bored with activity and has a very physical need to change activities frequently and after very short term periods.  I suggest you provide him with video games suitable for his age group.  This is an amazing technique that encourages the mind to focus on many factors at one time, such as eye hand coordination.  Allow him to play it as often as possible and I am sure you will see a difference in his abilities to stay focussed on one activity in a rather short term period.  We went through the same thing with our son and the video games have and continue to be an excellent tool to train the mind to stay focussed on one activity for longer periods of time.   Prayers and best wishes.

I'm 26, and I've got a whole apartment full of toys I never play with. I'll get really excited about something (e.g., books, musical instruments, gadgets), decide I have to have it, and then three days later I get bored with it, and it becomes just another thing taking up space in an already overcrowded apartment.

From my experience around kids that age, it doesn't matter if they have ADHD or not - they'll get really excited about a certain toy for a couple days, and then they'll move onto a new obsession. As they age, many kids grow out of this somewhat, but my ADHD definitely kept that from happening for me. I know he's only 4 now, but be careful about giving into his begging too much. Maybe you can help him learn the lesson I should've learned long ago.