What are the various Informix column data types?
The values in the coltype column of the syscolumns system catalog table are defined primarily in the Informix ESQL/C header sqltypes.h.  This header is used throughout the Informix server code.
The values in the coltype column are a mixture of an 8-bit (unsigned) integer in the low-order bits, and various flag values in the high-order bits.  In particular, when a column is defined with the NOT NULL qualifier, the 0x100 bit is set — which corresponds to 'add 256'.  There are other flag bits which you are less likely to see.
The value 4118 (decimal) shown in another answer corresponds to hex 0x1016; the 16 (aka 22 decimal) corresponds to SQLROW, and the 0x1000 (4096 decimal) bit corresponds to #define SQLNAMED 0x1000 /* Named row type vs row type */.  The type is (as noted in the other answer) a 'named row type'.
Consider a table:
CREATE TABLE bool_check
(
    b1      BOOLEAN NOT NULL,
    b2      BOOLEAN
);

The values in the sqltype column of syscolumns are:

b1 = 297 = 256 + 41
b2 = 41

These correspond to SQLUDTFIXED (type 41).  The type SQLBOOL is marked 'used by FE [front end], … not real major types in BE [back end, meaning database server]'.  The collength is 1 for both.
Relevant segments of the header include:
SQL Types:
#define SQLCHAR         0
#define SQLSMINT        1
#define SQLINT          2
#define SQLFLOAT        3
#define SQLSMFLOAT      4
#define SQLDECIMAL      5
#define SQLSERIAL       6
#define SQLDATE         7
#define SQLMONEY        8
#define SQLNULL         9
#define SQLDTIME        10
#define SQLBYTES        11
#define SQLTEXT         12
#define SQLVCHAR        13
#define SQLINTERVAL     14
#define SQLNCHAR        15
#define SQLNVCHAR       16
#define SQLINT8         17
#define SQLSERIAL8      18
#define SQLSET          19
#define SQLMULTISET     20
#define SQLLIST         21
#define SQLROW          22
#define SQLCOLLECTION   23
#define SQLROWREF       24
/*
 * Note: SQLXXX values from 25 through 39 are reserved to avoid collision
 *       with reserved PTXXX values in that same range. See p_types_t.h
 *
 * REFSER8: create tab with ref: referenced serial 8 rsam counter
 *      this is essentially a SERIAL8, but is an additional rsam counter
 *      this type only lives in the system catalogs and when read from
 *      disk is converted to SQLSERIAL8 with CD_REFSER8 set in ddcol_t 
 *      ddc_flags we must distinguish from SERIAL8 to allow both 
 *      counters in one tab
 *
 * SQLSTREAM: Is a synonym for SQLUDTFIXED used by CDR (Enterprise
 *      Replication) code
 */
#define SQLUDTVAR       40
#define SQLUDTFIXED     41
#define SQLSTREAM       SQLUDTFIXED
#define SQLREFSER8      42

/* These types are used by FE, they are not real major types in BE */
#define SQLLVARCHAR     43
#define SQLSENDRECV     44
#define SQLBOOL         45
#define SQLIMPEXP       46
#define SQLIMPEXPBIN    47

/* This type is used by the UDR code to track default parameters,
   it is not a real major type in BE */
#define SQLUDRDEFAULT   48
#define SQLUNKNOWN      51     
#define SQLBIGINT       52
#define SQLBIGSERIAL    53
#define SQLMAXTYPES     54

#define SQLLABEL        SQLINT

Flags:
#define SQLNONULL       0x0100  /* disallow nulls       */
/* a bit to show that the value is from a host variable */
#define SQLHOST         0x0200  /* Value is from host var. */
#define SQLNETFLT       0x0400  /* float-to-decimal for networked backend */
#define SQLDISTINCT     0x0800  /* distinct bit         */
#define SQLNAMED        0x1000  /* Named row type vs row type */
#define SQLDLVARCHAR    0x2000  /* Distinct of lvarchar */
#define SQLDBOOLEAN     0x4000  /* Distinct of boolean */
#define SQLCLIENTCOLL   0x8000  /* Collection is processed on client */

/* we are overloading SQLDBOOLEAN for use with row types */
#define SQLVARROWTYPE   0x4000  /* varlen row type */

There are also 'C-ISAM types' with numbers 100 to 125 and names such a CCHARTYPE and CDECIMALTYPE.  They are not of immediate concern here.  There are 524 lines in the header file (at least in the version I looked at). Of those, 74 are blank, 315 contain code, and the remaining lines are pure comment lines.  AFAIK, the SQLREFSER8 type is stillborn; it does not exist outside this file.
The types BLOB NOT NULL and CLOB NOT NULL are both encoded in coltype as 297 (41 + 256 — the same a BOOLEAN NOT NULL), or SQLUDTFIXED with collength of 72 (as opposed to 1 for BOOLEAN NOT NULL).  The fixed-length data is a descriptor that provides all the details about where the BLOB or CLOB value is actually stored.