Since they are new babies and you only have four of them, you might want to take a look at these two things while you are perusing the forums. I don't use a heat lamp of any kind, and many others don't either. Instead we use a heating pad "cave" for our chicks and they simply thrive. Natural day/night cycles, faster feathering because they are exposed to cooler temps around them, more natural because they dash under the heating pad for quick warm up but their entire environment isn't hot and most important, no risk of fire from the heat lamps or bulbs. I have no scientific proof to back up my theory here but pasty butt seems to be thing of the past....since they are sleeping during the night and not eating 24/7, I think that helps. Rarely, if ever, is pasty butt a problem with chicks raised under a broody hen, and this method mimics that closely.
The thread is Mama Heating Pad in the Brooder. This system works well in an inside brooder or outside.
There is a video of Mama Heating Pad at work on page 4 of that thread.
And this is how I raise my chicks....all of which are bright eyed, happy and healthy, and now laying very well. Your temps are warmer than mine - I raised 3 groups outside when temps were in the teens and twenties - so this should work very well for you. The heating pad we all use is the Sunbeam Xpress Heat, and with only 4 chicks you could easily use the smaller one that Walmart carries rather than having to order the larger one through Amazon. The only thing you have to be sure of is that there is switch that allows you to bypass the auto-off feature and set your heating pad to Stay On. Otherwise you'll be out there every hour or two to turn it back on.