I think it depends on what the current housemeates are told. If they're told the person they choose to go back in is eligible for winning then, if they have any sense (Richard certainly does), they would choose the person out of the 4 who is LEAST likely to win therefore not creating another rival/competitior.
If, however, they are not told that person re-entering is eligible to win, then they will choose their fave which could be NIkki or Lea (Glynn, RIchard and Pete all liked Lea)