I'm not sure what version of the HDMI cables you have, but to get a 4K signal, you need at least HDMI 1.4 or higher. Here is a breakdown of what each HDMI version supports:
In my opinion, I would buy all new cables with the exact length that you need and then rewire everything nicely. This way, you are future-proofing yourself in case you decide to upgrade in the next few years.
HDMI cables are a lot cheaper to buy online. One of the best brands at affordable prices that I like is Cable Matters. Their cables are high equality and hard to beat on price. Amazon carries them, or you can buy them directly. Three pack of gold plated HDMI cables only cost $11.49.
I don't see the need to buy super expensive HDMI cables. The signal is purely digital instead of being analog, so the signal either goes through or it doesn't. For analog, high-end materials matter, for digital not as much.
I've used Cable Matters for my LG and Samsung 4K TVs connected to Apple 4K TVs and never had an issue. I have a set of Ultra-Slim HDMI Cables and also standard ones. For durability, I would suggest going for thicker cables, but for travel and a more discrete setup, Ultra-Slim cables are the way to go.