Yeah, as Thunderbird said, most US network shows tend to broadcast original, first-run episodes during the three "sweeps" periods (November, February, May) of the season, where advertising rates are calculated. Enterprises' episode production order has also been cut from 26 episodes in the first two seasons, to 24 in the third season and now 22 in the fourth and final season, so UPN need to air more repeats to fill their schedule.
Enterprise also has a much more demanding post production schedule than most shows, due to the fairly heavy special-, optical- and visual effects requirement. IIRC, the typical lead-time for an episode of Enterprise is about six weeks from start of filming (7-10 days) to the final broadcast edit -- this doesn't factor in pre-production (choosing Directors/prepping, building additional sets, script rewrites, etc). If Enterprise' run wasn't interspersed with repeats due to sweeps, it would need to force periods of repeats at similar points anyway to allow post-production to catch-up, as their lead-time diminishes with each new original episode broadcast without the pause given by repeats.
AFAIK, starting with
4x17 Bound, the final batch of episodes should air consecutively over the next few weeks with the season finale
(4x21-22 ...These are the Voyages) airing on May 13th, 2005.
4x19 In A Mirror Darly Part 2 airs tonight and should have most trekkies in raptures with scenes of the crew in TOS uniforms, a further look at the Gorn and other TOS/Mirror Universe references.
Cheers,
BrynS