FF car seat for 4 yo

My son is RF in an Avionaut Stardust in my car, but he is about to outgrow the Joie iSpin 360 in his grandparents car. They are now wanting to FF him and are threatening to use an expired HBB even if I buy an ERF seat for their car. I think I am willing to concede and let them forward face him so I am looking for the best seat to do so. What seat do normal people use for their young 4 year olds? Advice in car seat groups seem to be HBB is better however is he too young for this? Should he still be harnessed?