You are correct with your assumings. Here are some details:
"Intel Streaming SIMD Extensions 4 (SSE4) introduces 54 new instructions in Intel 64 processors made from 45 nm process technology.
47 of the SSE4 instructions are available in 45 nm Intel processors based on the successor of Intel CoreTM microarchitecture (code named Penryn). This subset of 47 SSE4 instruction is referred to as SSE4.1 in this document.
SSE4.1 and seven other new SSE4 instructions are supported in 45 nm Intel processors based on a new microarchitecture (code named Nehalem). The subset of the 7 new SSE4 instructions available to Intel processors based on the Nehalem microarchitecture is referred to as SSE4.2 in this document."