The Ceralpha alpha-amylase assay reagent (R-CAAR4) cannot be used at pH 3-4.  However, you can perform a similar assay using blocked p-nitrophenyl maltoheptaoside (O-BPNPC7). Basically dissolve the BPNPC7 vial contents in 10 mL of water.  Incubate 0.2 mL of your enzyme (buffered) with 0.2 mL of BPNPC7 for 10 min at a set temperature.  Terminate the reaction by heating in a boiling water bath for 2 min.  Then add 0.2 mL of alpha-glucosidase (E-TSAGL; at 10 U/mL in 0.2 M Tris buffer, pH 7.0) and incubate for 10 min.  Add 3 mL stopping reagent (Trizma base pH 8.5) and read colour at 400 nm.

Product Page (R-CAAR4)