All around the country packs are being sold from Post Offices without the low value stamps issued on 8 March. This is a mistake in make-up by Royal Mail or their suppliers.
The price of the pack £5.10 includes the five low-value stamps worth 38p. If you have bought a pack with only 4 stamps in, go back to the Post Office or phone Post Office Customer Services to complain - (on 08457 22 33 44*. Lines are open from 8.15am - 6.00pm Monday to Friday and 8.30am - 2.00pm Saturday.)
Note that the stamps in the pack have been guillotined (possibly from coils) not torn from sheets. The backing paper is thus much smaller than on the sheet stamps. If your pack is missing the 1p - 20p stamps you want the guillotined ones to put in it, not stamps torn from sheets.