The all stages ones are not the best for what you want to do. You want a seat which goes up to age four, likely a spin seat, as these are popular. There are plenty of these which are isofix. The cheapest are the Joie 360 Spin, Graco Turn2Me, Graco Turn2Me i-size. All of these are around the £160 mark. I wouldn't touch anything cheaper TBH, as they are doing quite a complicated job so you don't want to go TOO cheap! But these have a good brand reputation/good ADAC test.
John Lewis has a good deal on the Cybex Sirona S2 i-size at the moment too, and the Joie i-Spin 360 GTi is reasonably priced.
You are right that the Every Stage FX only uses the isofix when forward facing.
Bear in mind that the ages given are always just approximate and based on average sized kids, so if your DC is taller than average they will outgrow sooner etc. Like clothing sizes. Therefore if you want to guarantee RF to age 4 these might be a bit of a gamble unless DC is small for their age.
There are rear facing seats for up to 7 years (again ish) which are not so pricey too and and aren't isofix, but they have a different belt path, so it isn't in the way. Though TBH not many cheaper options for these at the moment. Basically Britax Max Way Plus if you can find anyone with stock - Little Peas? Or Baby 2000 seem to have stock on the Britax Multi-Tech 3.
Attached are a couple of photos showing the belt path of these type of seats - not like the Joie ones. You do also have to thread tethers under the front seats.
You can't buy Klippan Kiss or Britax Two Way Elite any more unfortunately.