Can an ICF catalog (BCS) be defined greater than 4 GB?

As of V1R12, you can extend any catalog beyond the 4 GB limit. It must be defined as SMS managed and you must define it as an extended format BCS, which means it can use extended addressability.

You must specify extended format for a BCS by assigning an SMS data class, which indicates DSNTYPE=EXT parameter and subparameters R (meaning required) or P (meaning preferred) on the ISMF DATA CLASS DEFINE/ALTER panel. Use R to ensure the BCS is extended. The Extended Addressability value must be set to Y (Yes).

The catalog is still limited to single volume and maximum 123 extents, and since only extended format option available for a BCS is extended addressable. This means that BCSs cannot be compressed or striped.

Above is discussed in:

z/OS Managing Catalogs
Estimating Space Requirements for the BCS
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Estimating Space for an Extended Format BCS: A BCS is limited to 4 GB unless you define it as an extended format BCS, which means it can use extended addressability. Using extended addressability, the size limit for a BCS is determined by the control interval size multiplied by 4 GB. For example, a control interval size of 4 KB yields a maximum data set size of 16 TB, while a control interval size of 32 KB yields a maximum data set size of 128 TB.
No increase in processing time is expected for extended format data sets that grow beyond 4 GB. To use extended addressability, the BCS must be SMS managed and defined as extended format.
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z/OS DFSMS Managing Catalogs
Determining Catalog Size
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A catalog can have up to 123 extents but can only occupy space on a single volume.
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NOTE: You must ensure that you have APAR OA30000 installed if you have Catalogs defined as extended addressable and opened/accessed by systems using z/OS V1R10 or V1R11 will break. Catalog recovery is necessary to restore a broken catalog. This APAR OA30000 prevents extended addressable catalogs from being opened/accessed by systems that use z/OS V1R10 or V1R11.

See http://www-01.ibm.com/support/docview.wss?uid=isg1OA30000