first off : there is no such thing as FR4 MATERIAL ... Fr4 is a fire classification. it does not tell you anything about what board material is being used aprt from it being fire retardant category 4.
Laminates are made out of a fiberglass woven contruction and a resin. There are 3 kinds of fiberglass (E , NE and S) , each with their own specific dielectric properties and applications.
It is mainly the choice of fiberglass and the weave ( how the woven construction is made ) that determines the Er and Dk ( dielectric constant and dispersion ).
Materials made by Rogers (some people use the name 'Rogers' to indicate a type of pcb laminate. Rogers Corporation has at least 100 different laminates... This is the equivalent of answering 'Michelin' if someone asks you what car tires you need...) , Isola (Getek trademark) Nelco and others employ a fiberglass weave. The difference sits in the construction of the weave. ( how many strands per bundle , individual strand thickness , how tightly bundles are woven, how individal mats are stacked and what resin is used)
Prepreg is a laminate that is not fully cured. The fiberglass mat is impregnated with a resin that is half cured so it can be manipulated. if you re-heat it it will melt again. Only if you bring the resin above a certain temperature (the Tg temperature , also called the Glass Transition temperature ) will it solidify completely . ( there is also Td : decomposition. bring it above that and it starts falling apart ... )
For example FR408HR is an FR-4 grade material made by Isola. Tg is about 230 degree C. Td is 350 or so...
Perfect for RoHs compatible processes running at 220 ...260 degrees C.
The copper is bonded by chemically oxidizing the copper foil. They use a process that grows a copper oxide. that stuff is porous enough so that , during lamination , it will bond tightly with the resin.
I have a stack of prepreg and copper foils laying around somewhere at home. i'll take a coupe of pictures and post them here. that way you will see what is going on.
Your best solution would be to use a kapton based laminate. Those are flexible and you can solder them. For example a Dupont Pyralux. This uses a modified acrylic glue to make the copper stick to the Kapton foil. Any decent PCB fab will have Pyralux in stock as it is one of the common materials used for flex or semi-rigid pcb's.
If you need a sheet of copperfoil that has its back treated with the chemical process let me know.
If you want to do this at home you could go to a local pcb joint , ask to buy a sheet of pre-preg and copper foil and bond it in an autoclave... you will need a mold to apply pressure though.
And then exposure and etching becomes a bit of a problem.
I'd go with Pyralux. Ask the local pcb joint if you can buy a single sided or double sided flex core. get on ebay , buy a couple of sheets of dry-film and send it through a laminator , expose to uv and film and off you go...