From what I understand you want objects that are pickups. (I was thinking of something like touching them and then they'll either disappear and be placed in your inventory (Maybe showed by an UI object).
Or the players holds the item.
Once the objects are picked up you want to give the player the option to combine these objects, I was thinking about an UI popup that informs the player they can press a button an the object will be created.
I'm not sure what you mean with the progress bar, but creating any kind of bar in Unity isn't super hard and I can fill this bar with any percentage you want to.
On a side note, it's not needed to have 4 different prefabs for all of the objects, unless of course the models of these objects differentiate, then it's handy to use 4 different prefabs. Anyway, what you can do is attach a pickup script to the object that needs to be picked up and if written correctly this script is reusable for any kind of object.
I was also wondering how you'd like to combine them, either by placing them down somewhere or create them in the players 'inventory' and then place the created object.
And I'd like to know if the project is a 2D topdown game or 3rd/1st person game.
I hope you'll pick me for this project since I'm exited to do freelance Unity3D work.
Sorry if I have no reviews on my profile, that's because I've just joined this website and I'm new to this.
Greetings,
Feiko Joosten