Let's pretend that this was a corporation with, say, 100,000 shares. Your idea is that each of the four founders gets 5,000 shares each, and the other 80,000 shares will be used for investors, to compensate employees and so on. Implied in that is that the founders won't go below 5% each. (Unless the company amends its certificate of incorporation to authorize more shares, which might require the unanimous consent of the four founders.)
There's nothing wrong with that sort of setup. Heck, it's the typical way of doing thing if you ignore the parenthetical.
That's amazingly easy to do in an LLC operating agreement as well: "The company shall not issue additional membership interests if such issuance would result in any Founder's membership interest falling below 5% without the consent of such Founder."
As to your questions:
The company loses a little bit of flexibility, and runs into the problem that a single founder might be able to hold up a deal the company really needs. Also, you sort of set a bar for future investors, who may want similar provisions. The founder gets some certainty and control.
It depends on the management structure of the company. It's more common to require all managers to consent to admitting more members, which actually gives them more control.
Future investors will want the same thing, see above.
Conversion to corporation is easy -- just change to the structure I described in first paragraph. If that's really in the plan, then (depending on who your investors are) you might consider just starting off as an S-corp, to make the conversion easier. Taxation would be approximately the same.
That's just generalities. You should talk with your own attorney about your specific situation. (And, no, that's not me.)