LACP aggregates the links to a single logical link, but it doesn't provide more than a single physical members bandwidth to an individual host (e.g. aggregating 4x1G ethernet will only provide 1G max bandwidth for a single host). The additional bandwidth gains are provided by the load balancing of the servers/client across all of the links that make up the aggregate (see the '-b' option of the vif command). The other benefit of LACP is redundancy -- you can lose member links and not lose connectivity.
only one NDMP IP Session will never use 4 Gigabit Links. You have SRC-IP to Dst-IP, so 100MB/Sec is your limit.
Using 4 NDMP Sessions with only one IP , will again give you 100 MB/Sec if you run "-b ip" and SRC-DST IP on Cisco Switch. Switching to Port Based Load Balancing could help on Cisco and FAS site, as every session will have a different SRC and DST Port.
Given what has already been stated about LACP is true. Regardless of what your config is. The nature of aggregation is limited usually to a port per host.
An obvious question I have: Is why you using NDMP over ethernet instead of fibre channel? I assume you don't have FC capability.
Should the backup window be too great. I recommend you switch to fibre channel. As an example: Our backup windows went from 45+ hours (ethernet) to 9 hours (NDMP FC). I forget how many TB's of data it was but it's irrelevant.