Ikea Billy shelves (Tilekiln TN38)

I have an Ikea Billy bookshelf that needs more shelves as I only have two of the moveable ones. Colour is not important - they are 77 x 26 cm and have two cutouts on each end where they drop onto shelf supports. The hole shape is not important (it depends on the vintage of the Billy shelves) as I have a collection of random supports and can make them work. Any number from 1 to 4 would be great, Julia