[joyce c]

I have sown parsnips in a very deep 15" pot.  I've pulled out a couple and they look about 4" long and pencil thick.   Can I thin out and transplant them.  Many thanks. 

[arugula]

Yes you can, if you transplant them where you want them to be in the ground then they'll have plenty time to develop.

[JayG]

You can try as you have nothing to lose but they are considerably larger than when I transplant my thinnings (and that only works because my soil is very light so I can extract them easily without damage.)

If they are closer than about 5 inches you need to thin them out anyway whether you transplant the thinnings or not (and that's about the absolute minimum to get usable roots IMO.)