I have converted every one of my ceiling light fixtures to T91 LED plate lights from Home Depot. Such lights are like spots of quite-bright and natural sunshine. A 14.5 watt T91 is brighter and prettier than a 23 watt CFL bulb.

If you are asking then about T91 placement in a 4" junction box, I recommend using RACO 175 steel boxes. You must choose a deep box, and won't find more space in fiberglass boxes also suited for this, from Allied Moulded. There is limited space for wiring in the junction box, only perhaps for two leads.