The ladies have given you wonderful advice to ponder. I understand what you're saying about wishing to do one program, and I used to feel that way too! It really took some pondering and trying for me to realize that it is often easier to do more than one program (that is actually at the right level for each kiddo), then to tweak and change one program to meet such varying needs. The preschool stage versus the kindergarten ready-to-read stage are two very different stages. What a preschooler can do and what a school-age child can do creates a big divide between kiddos abilities. It's actually easier sometimes to combine as kiddos get older and have developed reading, writing, listening, and thinking skills.
So, with the ages of your kiddos, I would usually recommend staring LHFHG with your older child and doing it at the pace that suits him best (whether that be full-speed or half-speed). We did it at full-speed with one of my kiddos and at half-speed with my next one.
If you truly have your heart set on combining your little sweeties in LHTH, I would definitely add the 101 Favorite Bible stories and the older level devotional to LHTH and do the right side of LHFHG for your older child. Then, the following year, you'd begin LHFHG with your 4 year old, and your 6 year old would do the left side of LHFHG with the storytime box and the right side of the plans from Beyond for everything else.
Either plan will work. If your daughter seems like she'll be an early reader and writer and seems like a more "mature" listener as a student, then combining would be a viable option. If, however, your little one seems immature, on the young side, or a later bloomer, you'd be better off letting your son go ahead of her.
Take heart that even if you choose one path, and it seems like the other path would have been better, it is easily changed.

Sometimes only time shows the best path. You will know much better as you come to the end of LHTH what to do!
Blessings,
Carrie