I'm searching through laptops for a laptop with a hinge screen (something like Lenovo yoga or Asus flip).
I can't find any with a quad-core processor! They all come with a U processor.
Is there a reason for this? Would a HQ processor in a hybrid laptop be meaningless somehow? Or is it just to save power?
Yes, it is to save power. The Intel Processor U suffix stands for "Ultra-low power"
See my answer Meaning of Intel Processor suffixes for more information on how to compare processors for performance and power consumption.
Note also this comment by misha256 on the above answer:
A high spec mobile CPU can outperform a low spec desktop CPU no problem so the suffixes don't help you there. Likewise I've seen mobile CPUs that can burn through more watts than a low/mid range desktop CPU.
C - Desktop Processor based on the LGA 1150 package with high performance graphics
H - High performance graphics
K - Unlocked
M - Mobile
Q - Quad-core
R - Desktop processor based on BGA1364 (mobile) package with high performance graphics
S - Performance-optimized lifestyle
T - Power-optimized lifestyle
U - Ultra-low power
X - Extreme edition
Y - Extremely low power
The processor number is one of several factors, along with processor brand, specific system configurations, and system-level benchmarks, to be considered when choosing the right processor for your computing needs.
A higher number within a processor class or family generally indicates more features, but it may be more of one and less of another. Once you decide on a specific processor brand and type, compare processor numbers to verify the processor includes the features you are looking for.
See the following link for all the Letter and Product Line suffixes.
Source Intel® Processor Numbers: Laptop, Desktop, and Mobile Device
Yes, I believe it's to reduce power consumption, because that gives better battery life (or just lets them make the battery smaller and lighter) and also reduces the cooling needs. 2-in-1s need to be thin and light to be practical in tablet mode, so manufacturers don't want to have to shove a bunch of batteries, fans, and radiators into them.
