Oh don't worry, you don't need to be looking at next car seats yet, it's just that deal was too good to miss if anyone needed a spare or was on a tight budget.
@Smurf123 I have the exact same issue with DS2 - 17.7kg although not 4 until August! I moved him out of the Joie a while ago though.
You could change to a 25kg rear facing seat. That's the safest option and you'll get plenty of use out of it. It will likely last another 2-3 years and then you can pass it on to the baby as well. I decided not to do this because DH isn't really sold on RF and I was happy with 3 years RF and didn't particularly want to go out of my way to accommodate it for longer, but it is definitely worth doing if you're happy to. That could possibly even last you long enough that you only need one high backed booster seat between the two of them.
Or there are a couple of seats that harness forward facing. Joie Bold if you need the height in it. We have Britax Advansafix i-size, which I am really happy with. The harness height is about the same as the Joie so it won't keep working with the harness forever, but I think we'll get a few more months in it and the harness limit is 21kg so just gets you that bit of extra time after 18kg. It gets a better safety score than the Joie Bold, and I really like it as a high backed booster whereas the Bold only turns into an average booster (IMO) and is quite a bulky seat. The Joie Elevate only harnesses to 18kg (WTF Halfords!
) and it's such a flimsy seat anyway, I don't like it. Maybe it's worth a fit test in the Advansafix in a shop? We are right on the top harness slot and I don't know if it will last him until he is exactly 4 but I'm hoping to get to this summer at least.
You could keep going right up to the limit - it might take him longer than you think to put on that last 300g. Maybe research and pinpoint a couple of options you can pick up in a rush. The Joie i-Spin is suitable up to 19kg and they aren't that different as seats so although I wouldn't recommend overloading it, it's not likely to break as soon as he's 100g over or something.
I always think as with most things, there is nothing magic that changes overnight on their 4th birthday. If you would be happy putting him in a HBB at just turned 4 and you're just waiting for that deadline, then you may as well do it when he's 3y10m. On the other hand if you don't think he's mature enough for it yet, the opposite applies, he's not going to suddenly be more mature in 6 weeks' time, and it would probably be of benefit to get a longer lasting harnessed seat, whether you go for the Advansafix to gain ~6 months in a harness or Joie Bold/a 25kg rear facing seat to get essentially as long as you like. Again, you could try taking him to a car seat shop and doing a fit test in a high backed booster. When I worked in the shop the 3 year olds that were closer to 4 were generally OK about 80% of the time whereas there were a few who were going crazy wriggling, wrestling with the belt, complaining.... and if they were under 18kg, I really tried to steer them towards a seat with harness! Sometimes it worked and sometimes not. I didn't have any 25kg options to show them unfortunately. The Cybex boosters are nice and do fit younger/smaller children well although at 18kg that's probably not necessary. You might want to look at the HBBs with crotch strap like the Britax Kidfix ones.
I believe (I could be wrong) the thing about keeping them in a harness up to 4/5 comes from the US where the seats are quite different, people talk about pelvic development, but I've read the studies and they just talk about that pelvic development being a reason that a 3 year old needs a booster rather than a seatbelt alone. They are not arguing that a 3 year old needs a harness rather than a booster. So I don't know where it comes from.