The Easter holidays this year are Friday 29th March to Friday 12th April. I've just had a look at the Butlins website and you can book a four night stay for a family of four, Mon 8th-Fri 12th, at Skegness from £407 per unit in the basic accommodation going up to £690 in the better accommodation.
Or a three night stay, Fri 5th - Mon 8th, from £305 per unit up to £541.
In Minehead, a three night stay, Fri 5th - Mon 8th is from £212 per unit up to £705.
In Bognor, a three night stay, Fri 5th - Mon 8th is from £450 per unit up to £901.
EDIT
Where I live, schools finish on 19th July. I just had a look at the last weekend in July, Fri 26th - Mon 29th July for a family of four.
Skegness from £359 per unit to £757
Minehead from £323 per unit to £1,119
Bognor from £590 to £1,272
Or, depending when schools back in your area (here they'll go back on Wed 4th Sept 2024), you can also look at the first weekend in September, Fri 30th Aug - Mon 2 Sept,
Skegness from £341 per unit to £640
Minehead from £314 per unit to £1,260
Bognor from £442 per unit to £1,005
So, yes, you definitely can get a 3 or 4 night stay for £400 in Butlins - even in July or August.