See the led recessed lighting thread which has an infra red picture of a cfl in a recessed cab.

If you intend to use led over time, do not buy the trim ring as you will probably end up discarding it. The RT4 is under $40 @ Lowes.

There should be incentives from your local utility.

What is trim ring? Is it the trim? Can I just put a normal light bulb in the can without trim? The reason for not buying trim is because the led module (light bulb and trim together like cr4 at HD) that I could buy? What is rt4 at lowes? Is it 4" led module ?
Where could I find the cheapest 4" can? $20 for halo at HD. How about elco brand? Will cr4 fit this can?

Yes, it is the trim intended to hide the hole edges around the can. Light bulbs would work even without the trim.

If you install the CR4 or the RT4 from Sylvania and sold by Lowws, you will discard the trim ring.

There are quite a number of cans out on the market. You need to read the published specs for the chosen led module and can.

All this info is on the led recessed lighting thread.

Thank you so so much, Davidtay. I was going to buy the trim and light bulbs. I can use normal light bulb till I see good deals for led modules.