The Basic Numbers
All four players will be seeded in both Rome and Madrid, and will get first round BYEs. Therefore, the points they can earn (in each of the tournaments) are:
R2 loss - 10
R3 loss - 90
QF loss - 180
SF loss - 360
F loss - 600
W - 1000
David Ferrer & Rafael Nadal
First of all, it's important to note that Ferrer's Estoril results won't be relevant to these caculations - he already has an ATP250 tournament win (worth 250 points) that he can't count, so even if he wins Estoril his current points total won't change.
Stripping away the points Ferrer and Nadal are defending in the two clay Masters 1000 tournaments, Ferrer has 6380 points, and Nadal - 4895 points. That's a difference of 1485 points between the two, which Nadal will have to overcome in order to get to #4. Thus, we can start analyzing.
If Ferrer gets at least 515 points in both tournaments combined, he'll stay #4 until the French Open, regardless of Nadal's result. Thus, Ferrer will be #4 if:
- He reaches the final of either of the tournaments
- He makes at least a QF in one tournament and at least a SF in the other
- He wins either Madrid or Rome
- He reaches the final of one tournament and the SF of the other
- Make the finals at both tournaments
- Win one tournament and reach at least SF at the other
Of course, all of those scenarios can't all happen together - out of the four players, at most one can win each of the two tournaments. The draw in Madrid might even make some of these options unreachable. But until then, you can at least know what you want your favourite player to do.
Edited on May 9: Federer's loss means that he won't be seeded #2 in France; but at most #3 (can even fall down to #4 if Ferrer does really well in Rome and Federer does not).