5028 Duplo Primo Plate 4x4 Blue was a Service Packs exclusive set released in 1996.

Sets in Service Packs / Primo

Theme / Subtheme Service Packs / Primo
Year 1998
Pieces 4
Availability Retired
5022 LEGO Primo / Duplo Converter Bricks
Retail $3.99
Theme / Subtheme Service Packs / Primo
Year 1999
Pieces 2
Availability Retired
5021 LEGO Primo Wagons
Retail $3.99
Value $10.20
Growth 155.6%
Annual growth 4.4%