I feel like 4 is the weakest in the series. If you liked it, you will love 5.I haven't played FH5 yet, and while I'm not a big racing fan either Forza Horizon 4 ended up being one of my favorite games, well, ever. I gave it a shot on a whim and it definitely put its hooks in me almost right away. The graphics, sound, thrill of zipping around a beautiful map - it's all wonderfully crafted and just plain fun, and doesn't take itself too seriously. It plays arcadey enough for someone who isn't big into cars or racing to have a great time with it. It's also open world, so you're not stuck just running race after race like a more conventional racing game if you don't want to. It's all very accessible to the layman.
I'm not sure about the progression system of FH5 other than what I had read, but FH4 definitely had that feeling of "just one more race" to get a new ride or a wheel spin for prizes. FH5 is said to improve on that with accolades and skill points not being tied to a particular car, so I can imagine this one will have even a greater "hook" than the last one.