They will sit alongside the Shell petrol station, Greggs and Costa Coffee units already in place at the park.
However a fifth 3,982 sq ft unit remains available.
The final unit extends to approximately 370m2 (3,982ft2) and is available for occupation – say FHP Property Consultants.
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FHP are urging potential tenants to come forward and take up the empty space.
They said: “The final unit on this five unit scheme is a terrific opportunity for trade counter or even standard warehouse or industrial operators to lease prime new build space adjoining the A38 and within minutes of Junction 28 of the M1 itself.
"The strength of the location is clear from the fact that four of the units have been let already, with Howdens Joinery taking two and Screwfix taking two - and both should be up and running and trading in the very near future.”
The news comes as a further unit of 18,000 sq ft space has been built and sold to a local occupier – with more details to follow.
James Richards, development director at Clowes Developments, said: We are delighted at this latest deal, having lived and breathed Castlewood now for nearly 15 years.
"It’s terrific to see the huge range of quality occupiers we have and I’m sure the final unit on this scheme will be let quickly.
“We have news of other deals at Castlewood to bring forward shortly and I’m sure that this market-leading business park will continue to go from strength to strength as we reach the point of nearly having developed out the entirety of the scheme.”