If the piano now has #4 pins now, you could use a #6 pin to probably
get your "couple of years".  Your letter leads me to presume that you
already have the piano unstrung and the plate out of it.

Pinblock material is not that expensive.  Since you're already there
seems silly not to replace the block.  You will have to restring it now
and then later in the "couple of years".  That being said, and you
really feel the block is a gonner, why didn't you try the larger pins
or some doping before tearing the thing apart.

I have seen the plugs used and they don't work, or at least not well
enough for the effort.  What I have seen work is to leave the block in
there and rout out the section where the pin holes are and then replace
that with a solid chunk of new block, then carry on from there.

Sorry to say this but sounds a bit like a case of leaping before
looking, but best of luck.

John Hannaby