I would still keep the reg cab. You dont "have" to have a 4 door if you just have 1 child. Thats a luxury that has just become the norm in the last decade or so. We have 1 child. I had a 4 door GMC when we had him & traded it for a reg cab R/T. I pulled the console out & installed a jumpseat in it for lil man. I found that the extra room a 4 door provides was unused by me 99.99% of the time due to me driving so much for work & we just use the wife's vehicle when we are all together. I disliked having a 4 door truck being by myself most of the time. When my boy is w/ us I just strap his seat in w/ the normal seat belt, the back tension belt & the bottom car seat tension belt. I purchased the car seat hoops that are in the 4 doors & mounted them to the jump seat to strap the bottom car seat tension belt to. We just made a 2000 mile & 1 week round trip from Baton Rouge, La to Chattanooga, Tn then to Gatlinburg, Tn then To Nashville, Tn then back home last month w/ no issues & no complaints about lack of room. I do have a bed cover though to store everything in the bed.
Yes a 4 door will always be safer than a reg cab there is no denying that but a reg cab can/does work if you have a small child. I'm partial to reg cabs though. Thats all my dad ever had was reg cab 1 ton flat bed trucks & we would all pile in the truck & pull our camper all over the US for vacations. That was then though w/ alot less strict car seat laws. But like I said all these big SUV's & 4 door trucks are a luxury that has just became the norm over the yrs. If you like your reg cab then I would keep it