LUbuntu makes sense, you might get away with Ubuntu Mate but probably not. Lubuntu is not very pretty but works fine and probably is an easy learning curve if coming from an older windows. I suggest you steer clear of RedHat based distros, their strict adherence to Gnome 3 and its rules is not helpful and G3 is way beyond your P4
Partition your disk with a separate /home directory, that makes it easy to to do a fresh install, use the long term releases (18.04 right now) unless you enjoy upgrading/reinstalling. Ubuntu Mate and LUbuntu are good for 3 years from memory. Ubuntu has now dropped support for 32bit so if yours is a 32bit processor you will have issues beyond the life of 18.04.
Assuming you will be connected to the internet, be aware that an old, unsupported version of linux is not without risks.
At present, you can use the repository supplied FPC but you must install Lazarus from source, its easy and all documented on the wiki.
Davo