Your question actually asks the following two questions, which no one else seems to have answered directly, and I will try to do so:
"Will level 4 missions net me much more than level 3's?"
"How long would it take me to earn back my investment?"
Well, facts in your question indicate an estimated total BS cost of around 145-150 million isk. How long it will take you to earn that back is completely dependent on you, specifically in how skillful you (the player) are, which is totally subjective, but also in how ready you are to fly BSs.
As you can see from the following thread (http://www.eveonline.com/ingameboard.asp?a=topic&threadID=1333415), people talk about anything from around 10-100 million/hour. The better your skills, the faster it will go. If your support skills from flying a battleship are weak, I would recommend flying L4s with a friend or friends while you skill up, and blitzing them without worrying about salvage, because doing so will not be value-effective to your bottom line. Consider allowing a trusted friend to salvage your missions behind you for some kind of arrangement. In my opinion, the best mission running is with a friend.
As you support skills catch up to you, you can start to solo (if you wish, or you can still do it with friends) more safely and, thus, much more quickly, too. The faster your missions go, the more worthwhile salvaging them becomes. Salvaging can easily double your profit.