As with most articles written by this keyboard, one day my co-editor flippantly suggests a theme, and the next day an article appears in his inbox.
Standards required for a four-level preempt vary with position, vulnerability and partnership style. Some people prefer to play 4minor openings conventionally as stronger versions of 4major, giving more definition of those hands at the cost of some preemptive possibilities. Here, we assume all 4 level suit openings are natural, and assume that we have sufficient values to invite a slam, and ask what are the best methods to proceed?
The question is actually pretty simple, although it is remarkable how few people know the right answer! Over 4 minor, the most frequent thing you will want to do other than raising partner's suit or passing, is to play in no trumps (this is true both at pairs and teams). Thus 4NT is a natural response, asking for a sight of dummy. Equally you may well want to play in four of a major, which requires only ten tricks for game rather than eleven, thus four of a major is natural too. Difficult this, hey?!
`What is 4-P-4?', a partner e-mailed me a while back. `Roman keycard for clubs', I responded confidently. Several days later he told me he had done a simulation, and this was far more useful than using 4 as a cuebid or another cue-asking bid. So there!
Similarly, 5 should be used as RKCB over 4, albeit with modified responses - 5 showing 0 or 1. Let's face it - partner ought to have one of the top three honours for a four level opening!
Over 4 of a major suit it is easier still. Basically the same, except that 4NT is RKCB, as there is less mileage trying to make ten tricks in no trumps now, and five of a minor is a cue for the major (or natural constructive if you prefer - neither will come up very often anyway, and either is sensible). My only word of warning is to suggest caution over a four of a major opener - somehow the sight of partner opening at the four level often seems to drive people crazy - perfectly ordinary hands that may not be enough for game suddenly start leaping to slam!