KFW 55 credit multi-family house with 4 apartments - which loan is worthwhile?

Hello, the following situation:
Construction of 4 apartments for rent (total costs maybe 650 - 750 euros), offer from main bank 1.4% for 10 years vs. KFW 55 1.67% plus repayment subsidy 20. What additional costs can be expected for the KFW55 house during construction, which loan is worthwhile and if I have 400k KfW loan, how high is the interest rate for the remaining 250-350k? Thank you very much

No idea where you got the conditions from, but,

You get €100k per unit at 1.95% nominal for a term of 20 years with a fixed interest rate for 10 years without the option of special repayment and a provisioning period of 6 months. After all construction measures have been confirmed by your energy consultant for a KfW 55 house, including approval by the KfW, your loan will be reduced by €5k per unit.

Depending on the effort you want to put in, the energy consultant will cost you several thousand euros. In addition, you will have to make the ceilings, floors, windows, and exterior walls 30% better than the Energy Saving Ordinance 2016 reference value. In other words, you will have to invest more money in the above trades.

Since the credit-financing bank also passes on the KfW loans, the loan-to-value ratio will not change, meaning the other loans will not become cheaper.

In terms of your return, KfW 55 will not be worthwhile, unless you raise the additional costs on the rent.

An estimate is difficult without the information available. The subsidy will definitely be completely eaten up.

Taking out €400k and losing the special repayment option to get €20k in subsidies is totally unacceptable :-)