You need more RAM for 4K.
What processor do you have?
When you added your first clip to your timeline, was there a yellow-orange "render" line over the clip? This is an indication that your project settings did not match your video specs, which can results in very poor performance.
Although Rob's point is a good one too. MacAir's are built for portability, not power. You could well be underpowered for editing 4k. What processor does your laptop have?
thank you. I have a 1.4 GHz Intel Core i5 processor...
You are definitely underpowered for editing 4k video.