72 degrees at 4 weeks old is about right. You didnt mention what type of brooder though. If you have a small , little box, that is sealed up (covered in cardboard) then yes, that will insulate it and you will maintain the heat.
Mine on the other hand, are pretty massive compared to most. They also are wire bottomed, off the ground, and have a 3'x 3' wood bottomed box on 1 end of the 8 foot long brooder, the rest is all 1/4" hardware cloth wire, Even fully wrapped in plastic (what I do in the winter months) No low wattage bulb is going to do squat in that style of brooder. These style brooders are a hole lot different than the ol cardboard box deal most people use.
Either way, here are the general heat guidelines to adhere to for all chicks. Regardless of what it takes on your own personal brooder to achieve this, this should be maintained or you will risk chick loss.
Day 1-7 95 degrees
day 8-14 90
day 14-21 85
so on and so on, lowering by 5 degrees per week til you are around 65-70 after that no more heat should be needed, UNLESS it's this time of year and you are expecting EXTREME cold nights..