OK great that you're on board for forward facing until at least 4 :)
Most Group 1 seats don't have a separate base. It's a bit of a fad at the moment in the car seat companies to produce what they call "modular" systems where they put the Group 1 seat on a base that's the same as the baby seat base and then claim they are saving you money - when many Group 1 seats don't even require a separate base at all! Even isofix seats (e.g. Joie 360 Spin or Britax Dualfix) tend to have the isofix built in. And you can frequently buy an entire seat for less or the same amount as you'd be spending on the upgrade to your current seat, which would tie up the base that you then can't use for your new baby. It's a bit of a con IMO.
This brings me to question 2 - How big is your 14mo? If you know weight, height and/or clothing sizes, any of those would be brilliant with weight/height being best of all.
What you may not be aware of is that the next stage of seat comes in 2 sizes. The most common size like the Pearl (and variants) are up to 18kg or up to 105cm, depending on whether they are i-size or certified under the Group system. These are sold as up to 4 years, but that means that they will last an average child until somewhere around their fourth birthday, which may mean some time before they actually turn four. But you can also get hold of seats which will take a child up to 25kg rear facing, which is usually about 5-7 years, meaning you will definitely pass age 4, if that's your milestone you're aiming for.
Counterintuitively, the 25kg seats also tend to take up less room in the car as they sit more upright whereas 18kg seats are often quite reclined.
As you're planning to have a second child, you could also go for an 18kg seat now and reassess when DC2 is in need of DC1's toddler seat and/or DC1 is nearing the end of their time in the toddler seat, whether to move DC1 into a 25kg seat or high backed booster seat, or get a second 18kg seat for DC2.