Are you going to retire in the next four years? Or take all your cash out into gold, quit your job, and go into hiding for the next four years?

If neither apply to you, you can leave your money in your 401k. Selling index funds right now is ultimately up to you, but in the long term time in the market tends to beat timing the market.

What I’ve read in books is: Building up savings itself is more important than whether the returns are +10% or -10%, early in your career, since it will fluctuate but tends to average up.

And remember if your national index funds, bonds or whatever market and government backed investments lose half its value, you have bigger problems on your hand of the state of your country at that point which having cash on hand may or may not help anyway.