He is a cutie but definitely looks like a chi cross to me, especially in the second smaller pic with the full face shot. Body looks a little too stocky, and the muzzle is too broad and apears too short. Possibly a chi X pug?? How old and how big is he? Pics can be so deceiving as to size.
He looks to be purebred to me, you have to remember that chihuahuas come in all different shapes, sizes,coats, and colors. Google chihuahua pictures and you'll see many different looking "purebred" chihuahuas. :-)
Chihuahuas are like potatoe chips, you can't just have one!
I raise Chihuahuas and would say he is a Chi Mix. At 4 months he should weigh 3 pounds or less. I would like to see a picture of his parents. Were you able to see them when you purchased the dog? Per AKC standards a Chihuahua should weigh a maximum of 6 pounds. At 4 months his ears should be up. Sometimes the long coats may take a bit longer, but smooth coats ears come up pretty quick. Anyway, just love him for what he is...A cute, sweet puppy.