I was in a similar situation 1 month ago. DS1 will be 4 at the end of Jan, DS2 is 21 months and i needed to install a capsule for DD1. I purchased the Safe n Sound Hi Liner for DS1 with harness, it was a much slimmer seat and i needed so be able to fit 3 seats in a commodore wagon. I then transfered DS2 into the Maxi Rider which was DS1's and was able to also install a capsule for bub.

I thought i'd do the right thing and get them all professionaly installed especially with the seat that needed the harness because i had no idea how to do that. But when i was asked if DS was 18kg i had to say no not quite (i'm not good and telling fibs) and he wasn't going to be 4 until late Jan. So for a minute there the guy wasn't going to install the seat for me. He did change his mind thankfully as i'm not sure what we would have done then. But DS1 is fine in the Hi Liner and could almost be of regulation weight now anyway, DS2 is comfy in the Maxi Rider and DD is comfy in her capsule.

I'd say you'd be better off getting a booster of some kind for DS and then have your other little ones move up a seat (when it's time) you'll get use out of all of them and hopefully by the time DS is out of the booster maybe DD will be ready to go in it. Maybe That's what i'm hoping.