Lego Ninjago. It was a pretty appropriate choice for him, as he is into all things Lego and ninja right now (he is even enrolled in a Ninja Zone gymnastics class!).
This costume was pretty straightforward. I used Simplicity 1037, using views A, B, D, and F and made the size 4. I was really pleased with the fit-- I measured him and everything fits very true to size (a pleasant surprise compared to some of their adult patterns!).
A couple things to note about the pattern: It does not include any instructions/advice for the additional ties (arm/wrists and legs) pictured on the cover. That wasn't a big deal (and I ended up not using them), but I was fully expecting them to be included somehow as they are pictured. Also, the instructions for finishing the tabard piece (D) are pretty lacking... I fudged my way through the suggested bias binding, but it is far from pretty. But, you know, galloping horse and all that. (Thank goodness!)
The only modifications I made to the pattern were to add shoulder armor to the tabard and then create the headband (both self-drafted). I was pretty happy with how the shoulder armor turned out, but the weight of the armor pieces pulled the tabard backward (making them appear to be shoulder-blade armor-- doh!). So I added some velcro to the underside of the tabard shoulders to connect with the top of the robe shoulders in order to help hold everything in place. It helped some, but the fix was no match for my active ninja boy! He could easily defeat the velcro with all of his jumping-crouching ninja moves. But for trick-or-treating, it worked fine.
This was a fun costume to make.
And I know he is happy because he has asked to wear it every day since I finished it!