Can I merge my first 4 records to a single record with different LRECL ?.
1) The term "merge" (especially in SORTing environment) has a different meaning; the MERGE operation is applied to the process of merging records from several pre-sorted datasets/files into one sorted output dataset/file.
What you need is described better via the term "concatenate", or "append" several records in one single resulting record.
2) The value of LRECL is an attribute of created dataset; it has a very little relation to the operation of records concatenation; you are likely talking about concatenated record size.
LRECL of a dataset can be changed even without record concatenation, and vice versa: record concatenation can be performed while leaving the dataset LRECL unchanged (especially with RECFM=VB, but RECFM=FB too).