If you're going to Hawaii, do you really need a stroller? We were there recently with a 1YO and 3YO and we brought one umbrella stroller. Frankly, we only really needed it for the airport. With our airline, the child stroller gate checked for free. It did make things easier at the airport, but once in Hawaii, the older kid can walk and the younger kid can be carried easily enough.
As for renting a car-seat, I found this super frustrating. All the car rental places I've worked with have allowed you to "request" one to rent, but say they can't "guarantee" they will have one for you upon arrival. WHAT GOOD IS THAT?! Plus they cost extra to rent. Consequently, we lugged our own carseats.
Some helpful tips here - divide and conquer when possible. For example, I dropped everyone off at Departures with most of the gear (optionally rent one of those little carts/trolleys to schlep stuff to ticketing) then I parked the car and came back with the remaining 2 bags. Once you're in ticketing, the car-seats and big bags get checked and your load becomes much more manageable. So on check-in, depending on your airport, it may only be a grand total of 50 yards you have to schlep the car seats. Upon arrival, it can be similar - after retrieving all your bags at the carousel, one parent watches the kids & bags while the other one goes to pick up the rental car.
Another tip, though this is definitely luggage dependent. IF you have relatively stout luggage with wheels and a telescoping handle (can you picture this type?), you can thread the car-seat shoulder straps over the luggage handle so it just dangles down and hangs from your wheeled luggage. THEN, you can have your little one straddle the suitcase and ride it like a horse. They love it and you may end up transporting the little one, a car-seat, a bag, and still have one hand free. Granted, this is a little awkward, but it worked well for us. It's worth rehearsing at home before you get there. With our setup, I would have felt comfortable to wheel kid & car seat further than we actually had to. Your 4 year old should be able to walk on their own and possibly wheel a carry-on. They may enjoy the fun responsibility - make it seem like a big honor and act impressed.
Yet another tip - personally pack light. It's Hawaii, what all do you really need? A swim suit, flip flips, a couple changes of light clothes... am I missing anything?
Hope at least one of those tips helps for your particular situation. Have a great time.