They are a bit of a mess because old laws have been adapted to conform to more up to date knowledge on safety plus you've got two sets of legislation running in parallel at the moment. Yours aren't really young enough for the double set to worry you though as it's only really affecting baby and toddler seats at the minute.
Your 14 year old - Correct. They don't need a seat regardless of size, they are over 12.
Your 8yo:
Legally - She needs a car seat until she grows another 3cm, 135cm is the minimum to go without. The right car seat would be a high backed booster or a backless booster. Don't worry about weight even though these seats are rated to 36kg - this is essentially irrelevant, as long as she is not deforming the seat (unlikely) as the adult seatbelt will take her weight in a crash.
Realistically - Nobody is checking with a ruler - you could get away with not using one.
Ideally - You want to look at how the seatbelt fits her. If it is flat across her thighs, the shoulder part crosses her shoulder and collarbone, AND when she has her bum at the back of the seat, her knees can bend over the edge, then she's safe without any booster seat. However if any of these are true: The seatbelt rubs her neck, the lap belt lies across her tummy, she can't bend her knees over the edge when her bum is to the back of the seat (meaning she tends to shift her bum forwards to bend her knees) - then she is at increased risk of potentially serious injury without some kind of booster seat, and you should pick one which eliminates the fitting problems she has. A high back one will afford her some side impact protection, too. Some child/car combos will need a booster until 150cm to fit properly.
Your 4yo:
Legally - They need to be in an age appropriate car seat. They are in the weight class Group 2, which is either a high backed booster seat (not backless until 25kg AND 125cm minimum) or a rear-or-forward-facing seat which harnesses to 25kg.
What to know - People online will tell you that rear facing is safer - it is - but not sure I personally would justify buying a 25kg limit rear facing seat for a 4yo. But if you want to these seats do exist and you can find them. There also recently seems to be a lot of angst about keeping 4yos in a 5 point harness. Personally again I'm not sure I find these arguments compelling. In this case, there's no evidence that a 5 point harness forward facing is any safer than a seat which uses the adult seatbelt to restrain the child, and the behaviour argument about them needing to sit perfectly is subjective and could go both ways. Note that if you purchase a seat which is multi-stage and encompasses Group 1 (harnessed, up to 18kg) it is common that the harness only supports Group 1 and will be useless to you. If you're going to buy a high backed booster I would recommend you avoid these combination seats. However combination Group 2-3 is usual and fine.
Hope that helps.