Worth is something only YOU can determine.
To the extent that your games have a cpu limitation, you will do better.
Your 4790 has 8 threads and a passmark rating of 7204 with a single thread rating of 2227.
The 10400 has 12 threads and a rating of 12856/2604.
All good.
If your games are cpu centric like sims, mmo and strategy games, you will do better.
If you are playing fast action games, they will be more graphics limited
but your frame rate will be more consistent.
If you play multiplayer games, the extra threads will do you better.
Buy a 2 x 8gb DDR4 ram kit. 2666 is likely the max speed a non Z490 based motherboard can support.
Not a big negative since actual performance on Intel is not much impacted by ram speeds when you...