I've been playing FH4 and FH5 in parallel since FH5's release due to having had no Xbox during the main lifespan of FH4 (I bought my Series X a few months before FH5 release). The differences are readily apparent when you are playing them together every week.
For me, the main differentiators are:
- FH5 has slightly nicer driving physics. Particularly higher powered RWD cars feel more stable and controllable (which seems more realistic to me at least) in FH5.
- The FH5 map is noticeably larger and more varied, which allows for longer and more interesting routes (however I don't feel that we have enough routes currently - there is so much untapped potential in the map compared to earlier games and Eventlab is really hit and miss to find good routes using the extra roads)
- Visually FH5 has better lighting, much denser ground cover (for a more realistic look, particularly shading on ground cover gives the world a more solid feel). In addition, areas such as mud textures and even tarmac texture uniqueness are clearly better in FH5 than FH4. Every road looks more unique than they did in FH4.
- On the other hand, as a Northern European I actually prefer the less varied but greener scenery in FH4. Cactus deserts are fun for a bit, but give me rolling fields and treescapes any day.
- The seasons in FH4 are much more functional and interesting than FH5 - especially autumn and winter to contrast the greenness of spring and summer. Sure, they probably toned things down in FH5 because FH4's winter was not well liked online, but in the end I would prefer to have the snow than not.
- In terms of race types, I much prefer road racing in FH5 due to the improved physics. Off road racing is similar in both (slightly better feel in FH5, but I prefer the dirt and trees tracks in FH4). Cross country just feels a bit weird in FH5 with too many silly jumps and boring cactus fields, so I definitely prefer FH4's cross country.
- The drivatars are somewhat of a mixed bag. FH5 drivatars give you racing room better, so if you are closely racing them it's fun, however FH5 drivatars have a little too much rubber-banding and they launch a little too well. I'd prefer a more fair launch but slightly faster cornering skills like in FH4.
Still, there's absolutely no problem playing the two in parallel like this. Adjusting to the driving differences is easy, and being able to enjoy both maps adds variety and therefore longevity to the experience.