more difficult to start

[rh71]

almost 4 year old vehicle now and a couple times it didn't start up the first try. Didn't sound like it was straining so it shouldn't be a battery issue, although that battery has been going nearly 4 years already. It goes to the point where it's about to turn over then dies. This morning I had to try 4 times (with typical crank sound) before it finally turned over, but right after it still sounded weak for a brief moment. Drove ok, and I had driven it fine for 200mi. this past weekend.

Engine oil level is ok. I checked the manual and spark plugs are supposed to be a 100k mi. replacement item. I'm only at 21k. Yes it's a low mileage vehicle but have done the 3 annual maintenances on time. Any ideas? Gotta get all the wrinkles out before warranty expires in a few months.