yellow tips from heat stress?

There is something wrong there, it looks like a 5 day seedling should and 28°C is not that hot for the plant. My guess, you soil is very poor, is it commercial soil or did you use whatever was in you back yard?

When temperatures get too high, a number of warning signs will emerge. Leaves may show yellow or brown spotting, or they may actually appear burnt in places. Leaves may curl up or down or fold inward. When plants experience excessive heat for a long time, new growth patterns emerge. Another thing that can cause this is hot soil. Which means the nutrients in the soil are too strong for your seedling. Hopefully I answered your question and remember Happy Growing and Free your mind one puff at a time.

That's a really tiny plant for 4 weeks, my first guess is that your pH is out of range. The yellow cotyledons and slow growth kind of indicate to me that it's not getting the nutrients it needs to grow, and if it can't uptake from the soil because of pH, it'll start pulling nutrients from the lower leaves first. Heat could also be a factor, but I'd start by verifying your water is about 6.5 pH every time you water. Overwatering can also cause this since it robs the roots of oxygen. Good luck with your grow!

It could be, but at week 4, I'd be willing to bet this is her way of telling you to give her nutes. Specifically nitrogen.