"How do I choose? Can anybody give me some advice ? I will appreciate for it"
What's the 3850 stack not providing, which makes you believe you need to upgrade? Knowing that, might help in selecting a newer switch.
In theory, a fabric based switch might do better with East/West traffic, often found in a core device, than a stackable switch.
If you're looking long term, the Catalyst 9K series is likely better, as it's newer than the Catalys 4K series. That said, it's so new, adopting the Catalyst 9K now you might bump into some teething pains. The Catalyst 4K series, tough, is pretty old, which often generally implies stability. (NB: Seb mentions the Catalyst 4K series is considered an access layer switch, and although it's often used in that role, I wouldn't be concerned using it in a core role, and in times past, Cisco has sold the series as a carrier grade device. ) However, the Catalyst 9K series looks to be the logical replacement for the Catalyst 4K (and 6K?) series. It's architecture will allow "new" features in coming years.
BTW, you mentioned the 4506, but that only supports a single sup, so you'll have less redundancy than you have now with your stack of 3850s. Unless you're planning on obtaining two for your core, you might want to take that into consideration.
