Usually we use PacBio sequencing for genome study...we're talking about large genomic DNA of particular organism. Four plasmids of 40-60 kb = 160kb-240kb are relatively small. I would suggest:

1- primer walking

2- fragment those plasmids, clone those smaller fragments into cloning vector & sequence them using Sanger's method, which will be much more cheaper but a little bit laborious  

=)

http://allseq.com/need-sequencing/

this portal lists sequencing providers, where you can ask for a quotation. We found the one that suited us best with this

You will be probably better with Illumina sequencing on MiSeq. You might fragment the plasmids, put different indexes and sequence on the smallest flow cell available. You will get plenty of data.