This can be difficult enough, given all the variables, without spreading misinformation. No Sun framebuffer outputs a sync-on-green video signal. They output composite (H+V) sync, yes, but on its own line... not on green. Newer Suns have separate sync (H and V on individual lines), like VGA does. Monitors may support composite sync on green (3 BNC) without supporting ordinary composite sync (4 BNC), or vice versa.
It is possible that some unknown third-party display adapter for a Sun may output composite sync on green. But then it would not be compatible with a Sun monitor.
The previously mentioned NEC AccuSync will work, not because it has 5 BNC connectors (which means little), but because it shows this in the specs:
Input Signal
Video: Analog RGB 0.7 Vp-p/75 Ohms
Sync Separate Sync: TTL Level (Positive/Negative)
Composite Sync: TTL Level (Positive/Negative)
Composite Sync on Green: (0.3Vp-p negative 0.7Vp-p positive)
Note, separate declarations of separate sync (RGBHV), composite sync (RGB,H+V), and composite sync on green (R,G+H+V,B).