Hey ladyj,
Alot of this is subjective, but here goes. Some people do 1-2 peds electives their 4th year, others like myself did 5, in addition to 3 months in my 3rd year. I too was interested in Neonatology. Definitely should do a month of NICU, probably at the beginning of your 4th year. Food for thought: you may consider doing your peds residency at a place that has a Neonatology fellowship. They will have seen you during residency, and perhaps the director of the NICU can help. On the flip side of that, I have heard that sometimes it's better to go somewhere where there is no fellowship, b/c you'll have much more hands on during peds residency.
Aside from that, I think a Sub-I is helpful (I did 3), just b/c you see what it's like to be an intern, and pick up alot of pearls along the way. Of course, helpful to do Sub-I's at places you think you may want to go for residency. I also did Peds Nephro (just for kicks) and outpatient peds during 4th year. The one thing I had wish I had done during 4th year was a Peds ER or even PICU. But I did do all my peds stuff early on in my 4th year (to get LORs) and now am just cruising until Match day. I was also asked on many interviews what peds rotations I had done my 4th year and where, felt good to say 5!
Good luck! 😀