Unfortunately it's going to be hard to expect decent jumps in velocity with only 4 weeks to really train leading into the season. I definitely would suggest band work daily for arm health (and can lead to help in the long term velo process). A good long toss program is also something I would implement. My pitchers continue to use plyo care balls and med balls during the season (as part of a post pitching routine), but really aimed for arm maintenance and health. However these definitely can assist in the velocity process when used correctly too.
Hope this helps in some way!