M.S.
You can bring baby food/drinks through security. Just keep them within reason and declare them all as baby food (even if it's juice boxes and what looks like adult food), when you put your bag on the security belt. In my experience, food and drinks are your best friend when traveling with a toddler.
I have flown a LOT with my kids (I have a 2 year old and a 13 month old, I have mostly flown by myself with them). You will probably want your 18 month old to have his/her own seat for a 4 hour flight, and you will be best off if you use a carseat. We have flown with the Britax Marathon, and found it best to install rear facing so your child doesn't kick the seat in front of him/her. Talk to your child about the flight in advance, what the expectations are and stuff, even if you don't think s/he will understand.
Here's the other lessons I've learned:
Don't take an enormous amount of stuff on the plane.
Bring a few small toys and a few board books.
Bring a few "kryptonite items" (for us this includes a new hotwheel in the package, a super special treat like licorice or cookies, chocolate milk - I only take out the krypto items in a moment of desperation)
Don't worry if your child cries on the plane. It happens all the time and you paid for the flight just like everyone else!
Don't be afraid to ask for help from other passengers or flight attendants (you may not need help traveling with one child, but I constantly ask for help or take help that's offered)
Good luck! You can always msg me if you want more...like I said, I've done this a lot because my family lives far away.